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## Phase I — Linear Structures (Week 1)
<table>
<thead>
<tr>
<th align="left">Topic</th>
<th align="center">Set</th>
<th align="center">LC&nbsp;#</th>
<th align="left">Problem</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="9"><strong>1. Hash-Based Lookup</strong><br><sub>builds on nothing</sub></td>
<td align="center"><code>A</code></td><td align="center">217</td><td><a href="https://leetcode.com/problems/contains-duplicate/">Contains Duplicate</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">242</td><td><a href="https://leetcode.com/problems/valid-anagram/">Valid Anagram</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">1</td><td><a href="https://leetcode.com/problems/two-sum/">Two Sum</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">49</td><td><a href="https://leetcode.com/problems/group-anagrams/">Group Anagrams</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">347</td><td><a href="https://leetcode.com/problems/top-k-frequent-elements/">Top K Frequent Elements</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">383</td><td><a href="https://leetcode.com/problems/ransom-note/">Ransom Note</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">205</td><td><a href="https://leetcode.com/problems/isomorphic-strings/">Isomorphic Strings</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">128</td><td><a href="https://leetcode.com/problems/longest-consecutive-sequence/">Longest Consecutive Sequence</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">290</td><td><a href="https://leetcode.com/problems/word-pattern/">Word Pattern</a></td></tr>
<tr>
<td rowspan="5"><strong>2. Prefix Sum</strong><br><sub>builds on 1</sub></td>
<td align="center"><code>A</code></td><td align="center">303</td><td><a href="https://leetcode.com/problems/range-sum-query-immutable/">Range Sum Query</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">238</td><td><a href="https://leetcode.com/problems/product-of-array-except-self/">Product of Array Except Self</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">560</td><td><a href="https://leetcode.com/problems/subarray-sum-equals-k/">Subarray Sum Equals K</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">525</td><td><a href="https://leetcode.com/problems/contiguous-array/">Contiguous Array</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">974</td><td><a href="https://leetcode.com/problems/subarray-sums-divisible-by-k/">Subarray Sums Divisible by K</a></td></tr>
<tr>
<td rowspan="9"><strong>3. Two Pointers</strong><br><sub>builds on ordered iteration</sub></td>
<td align="center"><code>A</code></td><td align="center">125</td><td><a href="https://leetcode.com/problems/valid-palindrome/">Valid Palindrome</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">167</td><td><a href="https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/">Two Sum II</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">15</td><td><a href="https://leetcode.com/problems/3sum/">3Sum</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">11</td><td><a href="https://leetcode.com/problems/container-with-most-water/">Container With Most Water</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">42</td><td><a href="https://leetcode.com/problems/trapping-rain-water/">Trapping Rain Water</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">392</td><td><a href="https://leetcode.com/problems/is-subsequence/">Is Subsequence</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">977</td><td><a href="https://leetcode.com/problems/squares-of-a-sorted-array/">Squares of a Sorted Array</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">189</td><td><a href="https://leetcode.com/problems/rotate-array/">Rotate Array</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">80</td><td><a href="https://leetcode.com/problems/remove-duplicates-from-sorted-array-ii/">Remove Duplicates II</a></td></tr>
<tr>
<td rowspan="8"><strong>4. Sliding Window</strong><br><sub>builds on 3</sub></td>
<td align="center"><code>A</code></td><td align="center">121</td><td><a href="https://leetcode.com/problems/best-time-to-buy-and-sell-stock/">Best Time to Buy and Sell Stock</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">3</td><td><a href="https://leetcode.com/problems/longest-substring-without-repeating-characters/">Longest Substring Without Repeating Characters</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">424</td><td><a href="https://leetcode.com/problems/longest-repeating-character-replacement/">Longest Repeating Character Replacement</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">567</td><td><a href="https://leetcode.com/problems/permutation-in-string/">Permutation in String</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">76</td><td><a href="https://leetcode.com/problems/minimum-window-substring/">Minimum Window Substring</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">209</td><td><a href="https://leetcode.com/problems/minimum-size-subarray-sum/">Minimum Size Subarray Sum</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">1004</td><td><a href="https://leetcode.com/problems/max-consecutive-ones-iii/">Max Consecutive Ones III</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">643</td><td><a href="https://leetcode.com/problems/maximum-average-subarray-i/">Maximum Average Subarray</a></td></tr>
<tr>
<td rowspan="9"><strong>5. Binary Search</strong><br><sub>builds on ordered / monotonic invariants</sub></td>
<td align="center"><code>A</code></td><td align="center">704</td><td><a href="https://leetcode.com/problems/binary-search/">Binary Search</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">74</td><td><a href="https://leetcode.com/problems/search-a-2d-matrix/">Search a 2D Matrix</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">875</td><td><a href="https://leetcode.com/problems/koko-eating-bananas/">Koko Eating Bananas</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">33</td><td><a href="https://leetcode.com/problems/search-in-rotated-sorted-array/">Search in Rotated Sorted Array</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">153</td><td><a href="https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/">Find Minimum in Rotated Sorted Array</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">35</td><td><a href="https://leetcode.com/problems/search-insert-position/">Search Insert Position</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">162</td><td><a href="https://leetcode.com/problems/find-peak-element/">Find Peak Element</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">34</td><td><a href="https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/">First &amp; Last Position</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">4</td><td><a href="https://leetcode.com/problems/median-of-two-sorted-arrays/">Median of Two Sorted Arrays</a></td></tr>
<tr>
<td rowspan="5"><strong>6. Stack</strong><br><sub>independent primitive</sub></td>
<td align="center"><code>A</code></td><td align="center">20</td><td><a href="https://leetcode.com/problems/valid-parentheses/">Valid Parentheses</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">155</td><td><a href="https://leetcode.com/problems/min-stack/">Min Stack</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">150</td><td><a href="https://leetcode.com/problems/evaluate-reverse-polish-notation/">Evaluate Reverse Polish Notation</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">71</td><td><a href="https://leetcode.com/problems/simplify-path/">Simplify Path</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">224</td><td><a href="https://leetcode.com/problems/basic-calculator/">Basic Calculator</a></td></tr>
<tr>
<td rowspan="5"><strong>7. Monotonic Stack</strong><br><sub>builds on 6</sub></td>
<td align="center"><code>A</code></td><td align="center">739</td><td><a href="https://leetcode.com/problems/daily-temperatures/">Daily Temperatures</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">853</td><td><a href="https://leetcode.com/problems/car-fleet/">Car Fleet</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">496</td><td><a href="https://leetcode.com/problems/next-greater-element-i/">Next Greater Element I</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">901</td><td><a href="https://leetcode.com/problems/online-stock-span/">Online Stock Span</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">84</td><td><a href="https://leetcode.com/problems/largest-rectangle-in-histogram/">Largest Rectangle in Histogram</a></td></tr>
</tbody>
</table>
---
## Phase II — Nodal &amp; Grid Structures (Week 2)
<table>
<thead>
<tr>
<th align="left">Topic</th>
<th align="center">Set</th>
<th align="center">LC&nbsp;#</th>
<th align="left">Problem</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="10"><strong>8. Linked List</strong><br><sub>independent primitive</sub></td>
<td align="center"><code>A</code></td><td align="center">206</td><td><a href="https://leetcode.com/problems/reverse-linked-list/">Reverse Linked List</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">21</td><td><a href="https://leetcode.com/problems/merge-two-sorted-lists/">Merge Two Sorted Lists</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">141</td><td><a href="https://leetcode.com/problems/linked-list-cycle/">Linked List Cycle</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">19</td><td><a href="https://leetcode.com/problems/remove-nth-node-from-end-of-list/">Remove Nth Node From End</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">143</td><td><a href="https://leetcode.com/problems/reorder-list/">Reorder List</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">876</td><td><a href="https://leetcode.com/problems/middle-of-the-linked-list/">Middle of the Linked List</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">92</td><td><a href="https://leetcode.com/problems/reverse-linked-list-ii/">Reverse Linked List II</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">82</td><td><a href="https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/">Remove Duplicates from Sorted List II</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">61</td><td><a href="https://leetcode.com/problems/rotate-list/">Rotate List</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">86</td><td><a href="https://leetcode.com/problems/partition-list/">Partition List</a></td></tr>
<tr>
<td rowspan="4"><strong>9. Hybrid Structures</strong><br><sub>builds on 1 + 8</sub></td>
<td align="center"><code>A</code></td><td align="center">146</td><td><a href="https://leetcode.com/problems/lru-cache/">LRU Cache</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">2</td><td><a href="https://leetcode.com/problems/add-two-numbers/">Add Two Numbers</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">138</td><td><a href="https://leetcode.com/problems/copy-list-with-random-pointer/">Copy List with Random Pointer</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">380</td><td><a href="https://leetcode.com/problems/insert-delete-getrandom-o1/">Insert Delete GetRandom O(1)</a></td></tr>
<tr>
<td rowspan="5"><strong>10. Matrix Index Math</strong><br><sub>builds on 3 — NOT graph traversal</sub></td>
<td align="center"><code>A</code></td><td align="center">48</td><td><a href="https://leetcode.com/problems/rotate-image/">Rotate Image</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">54</td><td><a href="https://leetcode.com/problems/spiral-matrix/">Spiral Matrix</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">73</td><td><a href="https://leetcode.com/problems/set-matrix-zeroes/">Set Matrix Zeroes</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">36</td><td><a href="https://leetcode.com/problems/valid-sudoku/">Valid Sudoku</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">289</td><td><a href="https://leetcode.com/problems/game-of-life/">Game of Life</a></td></tr>
</tbody>
</table>
---
## Phase III — Hierarchical Structures (Week 3)
<table>
<thead>
<tr>
<th align="left">Topic</th>
<th align="center">Set</th>
<th align="center">LC&nbsp;#</th>
<th align="left">Problem</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="10"><strong>11. Tree DFS</strong><br><sub>builds on recursion</sub></td>
<td align="center"><code>A</code></td><td align="center">226</td><td><a href="https://leetcode.com/problems/invert-binary-tree/">Invert Binary Tree</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">104</td><td><a href="https://leetcode.com/problems/maximum-depth-of-binary-tree/">Maximum Depth of Binary Tree</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">100</td><td><a href="https://leetcode.com/problems/same-tree/">Same Tree</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">572</td><td><a href="https://leetcode.com/problems/subtree-of-another-tree/">Subtree of Another Tree</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">236</td><td><a href="https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/">Lowest Common Ancestor</a></td></tr>
<tr><td align="center"><code>A</code> ⭐</td><td align="center">124</td><td><a href="https://leetcode.com/problems/binary-tree-maximum-path-sum/">Binary Tree Maximum Path Sum</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">101</td><td><a href="https://leetcode.com/problems/symmetric-tree/">Symmetric Tree</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">112</td><td><a href="https://leetcode.com/problems/path-sum/">Path Sum</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">129</td><td><a href="https://leetcode.com/problems/sum-root-to-leaf-numbers/">Sum Root to Leaf Numbers</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">543</td><td><a href="https://leetcode.com/problems/diameter-of-binary-tree/">Diameter of Binary Tree</a></td></tr>
<tr>
<td rowspan="5"><strong>12. Tree BFS</strong><br><sub>builds on 6's sibling (queue) + 11</sub></td>
<td align="center"><code>A</code></td><td align="center">102</td><td><a href="https://leetcode.com/problems/binary-tree-level-order-traversal/">Level Order Traversal</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">199</td><td><a href="https://leetcode.com/problems/binary-tree-right-side-view/">Right Side View</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">1448</td><td><a href="https://leetcode.com/problems/count-good-nodes-in-binary-tree/">Count Good Nodes</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">103</td><td><a href="https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/">Zigzag Level Order</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">637</td><td><a href="https://leetcode.com/problems/average-of-levels-in-binary-tree/">Average of Levels</a></td></tr>
<tr>
<td rowspan="7"><strong>13. Binary Search Tree</strong><br><sub>builds on 5 + 11</sub></td>
<td align="center"><code>A</code></td><td align="center">98</td><td><a href="https://leetcode.com/problems/validate-binary-search-tree/">Validate BST</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">230</td><td><a href="https://leetcode.com/problems/kth-smallest-element-in-a-bst/">Kth Smallest in BST</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">235</td><td><a href="https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/">LCA of BST</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">105</td><td><a href="https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/">Build from Preorder + Inorder</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">530</td><td><a href="https://leetcode.com/problems/minimum-absolute-difference-in-bst/">Minimum Absolute Difference</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">173</td><td><a href="https://leetcode.com/problems/binary-search-tree-iterator/">BST Iterator</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">108</td><td><a href="https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/">Sorted Array to BST</a></td></tr>
<tr>
<td rowspan="9"><strong>14. Heap / Priority Queue</strong><br><sub>implicit tree over array</sub></td>
<td align="center"><code>A</code></td><td align="center">215</td><td><a href="https://leetcode.com/problems/kth-largest-element-in-an-array/">Kth Largest in Array</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">703</td><td><a href="https://leetcode.com/problems/kth-largest-element-in-a-stream/">Kth Largest in Stream</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">973</td><td><a href="https://leetcode.com/problems/k-closest-points-to-origin/">K Closest Points</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">621</td><td><a href="https://leetcode.com/problems/task-scheduler/">Task Scheduler</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">23</td><td><a href="https://leetcode.com/problems/merge-k-sorted-lists/">Merge K Sorted Lists</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">1046</td><td><a href="https://leetcode.com/problems/last-stone-weight/">Last Stone Weight</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">373</td><td><a href="https://leetcode.com/problems/find-k-pairs-with-smallest-sums/">K Pairs with Smallest Sums</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">295</td><td><a href="https://leetcode.com/problems/find-median-from-data-stream/">Find Median from Data Stream</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">502</td><td><a href="https://leetcode.com/problems/ipo/">IPO</a></td></tr>
<tr>
<td rowspan="3"><strong>15. Trie</strong><br><sub>builds on 11</sub></td>
<td align="center"><code>A</code></td><td align="center">208</td><td><a href="https://leetcode.com/problems/implement-trie-prefix-tree/">Implement Trie</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">211</td><td><a href="https://leetcode.com/problems/design-add-and-search-words-data-structure/">Add &amp; Search Words</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">212</td><td><a href="https://leetcode.com/problems/word-search-ii/">Word Search II</a> <sub>(needs topic 20 — schedule after)</sub></td></tr>
</tbody>
</table>
---
## Phase IV — Relational Structures (Week 4)
<table>
<thead>
<tr>
<th align="left">Topic</th>
<th align="center">Set</th>
<th align="center">LC&nbsp;#</th>
<th align="left">Problem</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="7"><strong>16. Graph DFS</strong><br><sub>builds on 11 + visited set</sub></td>
<td align="center"><code>A</code></td><td align="center">200</td><td><a href="https://leetcode.com/problems/number-of-islands/">Number of Islands</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">695</td><td><a href="https://leetcode.com/problems/max-area-of-island/">Max Area of Island</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">133</td><td><a href="https://leetcode.com/problems/clone-graph/">Clone Graph</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">417</td><td><a href="https://leetcode.com/problems/pacific-atlantic-water-flow/">Pacific Atlantic Water Flow</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">130</td><td><a href="https://leetcode.com/problems/surrounded-regions/">Surrounded Regions</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">399</td><td><a href="https://leetcode.com/problems/evaluate-division/">Evaluate Division</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">261</td><td><a href="https://leetcode.com/problems/graph-valid-tree/">Graph Valid Tree</a></td></tr>
<tr>
<td rowspan="5"><strong>17. Graph BFS</strong><br><sub>builds on 12 + visited set — shortest path unweighted</sub></td>
<td align="center"><code>A</code></td><td align="center">994</td><td><a href="https://leetcode.com/problems/rotting-oranges/">Rotting Oranges</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">127</td><td><a href="https://leetcode.com/problems/word-ladder/">Word Ladder</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">909</td><td><a href="https://leetcode.com/problems/snakes-and-ladders/">Snakes and Ladders</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">433</td><td><a href="https://leetcode.com/problems/minimum-genetic-mutation/">Minimum Genetic Mutation</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">286</td><td><a href="https://leetcode.com/problems/walls-and-gates/">Walls and Gates</a></td></tr>
<tr>
<td rowspan="4"><strong>18. Topological Sort</strong><br><sub>builds on 16</sub></td>
<td align="center"><code>A</code></td><td align="center">207</td><td><a href="https://leetcode.com/problems/course-schedule/">Course Schedule</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">210</td><td><a href="https://leetcode.com/problems/course-schedule-ii/">Course Schedule II</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">269</td><td><a href="https://leetcode.com/problems/alien-dictionary/">Alien Dictionary</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">1462</td><td><a href="https://leetcode.com/problems/course-schedule-iv/">Course Schedule IV</a></td></tr>
<tr>
<td rowspan="4"><strong>19. Union-Find</strong><br><sub>builds on 16's problem class</sub></td>
<td align="center"><code>A</code></td><td align="center">684</td><td><a href="https://leetcode.com/problems/redundant-connection/">Redundant Connection</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">323</td><td><a href="https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/">Number of Connected Components</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">128</td><td><a href="https://leetcode.com/problems/longest-consecutive-sequence/">Longest Consecutive (union-find variant)</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">721</td><td><a href="https://leetcode.com/problems/accounts-merge/">Accounts Merge</a></td></tr>
</tbody>
</table>
---
## Phase V — Decision-Space Exploration (Weeks 4–5)
<table>
<thead>
<tr>
<th align="left">Topic</th>
<th align="center">Set</th>
<th align="center">LC&nbsp;#</th>
<th align="left">Problem</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="8"><strong>20. Backtracking</strong><br><sub>builds on 16 — then unlock LC 212 from topic 15</sub></td>
<td align="center"><code>A</code></td><td align="center">78</td><td><a href="https://leetcode.com/problems/subsets/">Subsets</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">39</td><td><a href="https://leetcode.com/problems/combination-sum/">Combination Sum</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">46</td><td><a href="https://leetcode.com/problems/permutations/">Permutations</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">79</td><td><a href="https://leetcode.com/problems/word-search/">Word Search</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">17</td><td><a href="https://leetcode.com/problems/letter-combinations-of-a-phone-number/">Letter Combinations</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">77</td><td><a href="https://leetcode.com/problems/combinations/">Combinations</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">22</td><td><a href="https://leetcode.com/problems/generate-parentheses/">Generate Parentheses</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">52</td><td><a href="https://leetcode.com/problems/n-queens-ii/">N-Queens II</a></td></tr>
<tr>
<td rowspan="9"><strong>21. 1-D Dynamic Programming</strong><br><sub>builds on 20</sub></td>
<td align="center"><code>A</code></td><td align="center">70</td><td><a href="https://leetcode.com/problems/climbing-stairs/">Climbing Stairs</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">746</td><td><a href="https://leetcode.com/problems/min-cost-climbing-stairs/">Min Cost Climbing Stairs</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">198</td><td><a href="https://leetcode.com/problems/house-robber/">House Robber</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">213</td><td><a href="https://leetcode.com/problems/house-robber-ii/">House Robber II</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">322</td><td><a href="https://leetcode.com/problems/coin-change/">Coin Change</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">300</td><td><a href="https://leetcode.com/problems/longest-increasing-subsequence/">Longest Increasing Subsequence</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">139</td><td><a href="https://leetcode.com/problems/word-break/">Word Break</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">91</td><td><a href="https://leetcode.com/problems/decode-ways/">Decode Ways</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">647</td><td><a href="https://leetcode.com/problems/palindromic-substrings/">Palindromic Substrings</a></td></tr>
<tr>
<td rowspan="8"><strong>22. Multi-D / Grid DP</strong><br><sub>builds on 21</sub></td>
<td align="center"><code>A</code></td><td align="center">62</td><td><a href="https://leetcode.com/problems/unique-paths/">Unique Paths</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">64</td><td><a href="https://leetcode.com/problems/minimum-path-sum/">Minimum Path Sum</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">1143</td><td><a href="https://leetcode.com/problems/longest-common-subsequence/">Longest Common Subsequence</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">120</td><td><a href="https://leetcode.com/problems/triangle/">Triangle</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">63</td><td><a href="https://leetcode.com/problems/unique-paths-ii/">Unique Paths II</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">72</td><td><a href="https://leetcode.com/problems/edit-distance/">Edit Distance</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">221</td><td><a href="https://leetcode.com/problems/maximal-square/">Maximal Square</a></td></tr>
<tr><td align="center"><code>B</code> ⭐</td><td align="center">97</td><td><a href="https://leetcode.com/problems/interleaving-string/">Interleaving String</a></td></tr>
<tr>
<td rowspan="7"><strong>23. Greedy</strong><br><sub>builds on 21 — includes Kadane's</sub></td>
<td align="center"><code>A</code></td><td align="center">53</td><td><a href="https://leetcode.com/problems/maximum-subarray/">Maximum Subarray</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">55</td><td><a href="https://leetcode.com/problems/jump-game/">Jump Game</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">45</td><td><a href="https://leetcode.com/problems/jump-game-ii/">Jump Game II</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">134</td><td><a href="https://leetcode.com/problems/gas-station/">Gas Station</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">122</td><td><a href="https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/">Best Time to Buy and Sell Stock II</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">918</td><td><a href="https://leetcode.com/problems/maximum-sum-circular-subarray/">Maximum Sum Circular Subarray</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">763</td><td><a href="https://leetcode.com/problems/partition-labels/">Partition Labels</a></td></tr>
<tr>
<td rowspan="7"><strong>24. Intervals</strong><br><sub>builds on 23</sub></td>
<td align="center"><code>A</code></td><td align="center">57</td><td><a href="https://leetcode.com/problems/insert-interval/">Insert Interval</a></td>
</tr>
<tr><td align="center"><code>A</code></td><td align="center">56</td><td><a href="https://leetcode.com/problems/merge-intervals/">Merge Intervals</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">435</td><td><a href="https://leetcode.com/problems/non-overlapping-intervals/">Non-Overlapping Intervals</a></td></tr>
<tr><td align="center"><code>A</code></td><td align="center">253</td><td><a href="https://leetcode.com/problems/meeting-rooms-ii/">Meeting Rooms II</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">228</td><td><a href="https://leetcode.com/problems/summary-ranges/">Summary Ranges</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">452</td><td><a href="https://leetcode.com/problems/minimum-number-of-arrows-to-burst-balloons/">Minimum Arrows to Burst Balloons</a></td></tr>
<tr><td align="center"><code>B</code></td><td align="center">252</td><td><a href="https://leetcode.com/problems/meeting-rooms/">Meeting Rooms I</a></td></tr>
</tbody>
</table>
## Dependency Spine
<p align="center">
<img alt="Dependency graph of the 24 topics across five phases, showing which topics build on which" src=".github/assets/dependency-spine.svg" />
</p>
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