feat(islands): add StudyDeck component and markdown decks for algorithms

islands/decks/hashing.md

@@ -0,0 +1,72 @@
+## Hashing
+
+Trade O(n) memory for **O(1) average lookup**. The workhorse behind "have I seen this before?".
+
+---
+
+## Recognition
+
+- "Does X exist?" / "How many times?" → `Set` / `Map`
+- Pair matching (complement lookup) → map value → index
+- Grouping by a derived key → map key → bucket
+- Counting characters → frequency map
+
+---
+
+## Template — complement lookup (LC 1)
+
+```js
+function twoSum(nums, target) {
+  const seen = new Map(); // value -> index
+  for (let i = 0; i < nums.length; i++) {
+    const need = target - nums[i];
+    if (seen.has(need)) return [seen.get(need), i];
+    seen.set(nums[i], i);
+  }
+}
+```
+
+One pass: check for the partner **before** inserting yourself.
+
+---
+
+## Template — grouping (LC 49)
+
+```js
+function groupAnagrams(strs) {
+  const groups = new Map();
+  for (const s of strs) {
+    const key = [...s].sort().join("");
+    if (!groups.has(key)) groups.set(key, []);
+    groups.get(key).push(s);
+  }
+  return [...groups.values()];
+}
+```
+
+The whole trick is **choosing the canonical key**.
+
+---
+
+## Pitfalls
+
+- Frequency compare needs both directions (surplus *and* deficit)
+- Object keys coerce to strings — use `Map` for numbers
+- Hashing is O(1) *average*; keys must be cheap to compute
+
+---
+
+## Recall
+
+- Two Sum: why insert after the lookup, not before?
+- Group Anagrams: name two valid canonical keys
+- LC 128 Longest Consecutive: why check `!set.has(x - 1)` first?
+
+---
+
+## Drill
+
+- 217 Contains Duplicate · 242 Valid Anagram
+- 1 Two Sum · 49 Group Anagrams
+- 347 Top K Frequent · 383 Ransom Note
+- 205 Isomorphic Strings · 128 Longest Consecutive · 290 Word Pattern