docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(linked-list)/2-add-two-numbers.mdx

@@ -27,6 +27,20 @@ sidebar:
 - `0 <= Node.val <= 9`
 - It is guaranteed that the list represents a number that does not have leading zeros.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["addTwoNumbers(l1, l2)"]) --> I["dummy = ListNode(), carry = 0, curr = dummy — dummy.next will be the head"]
+  I --> W{"l1 or l2 or carry?"}
+  W -- no --> E(["return dummy.next"])
+  W -- yes --> A["s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry — a missing digit counts as 0"]
+  A --> D["carry, val = divmod(s, 10)"]
+  D --> N["curr.next = ListNode(val), curr = curr.next"]
+  N --> V["l1 = l1.next if l1 else None, l2 = l2.next if l2 else None"]
+  V --> W
+```
+
 ## Solution
 
 ```py
@@ -55,3 +69,7 @@ class Solution:
 
         return dummy.next
 ```
+
+## Explanation
+
+<YouTube id="wgFPrzTjm7s" title="Add Two Numbers - Leetcode 2 - Python" />

apps/docs/content/(linked-list)/23-merge-k-sorted-lists.mdx

@@ -30,6 +30,27 @@ sidebar:
 - `lists[i]` is sorted in ascending order.
 - The sum of lists[i].length will not exceed 10^4.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["mergeKLists(lists)"]) --> I["heap = []"]
+  I --> F{"more i, node in enumerate(lists)?"}
+  F -- yes --> C{"node?"}
+  C -- yes --> P["heapq.heappush(heap, (node.val, i, node)) — i breaks ties so nodes are never compared"]
+  P --> F
+  C -- no --> F
+  F -- no --> D["D = ListNode(), cur = D"]
+  D --> W{"heap?"}
+  W -- no --> E(["return D.next"])
+  W -- yes --> O["val, i, node = heapq.heappop(heap) — the smallest head across all lists"]
+  O --> L["cur.next = node, cur = node, node = node.next"]
+  L --> N{"node?"}
+  N -- yes --> Q["heapq.heappush(heap, (node.val, i, node))"]
+  Q --> W
+  N -- no --> W
+```
+
 ## Solution
 
 ```py
@@ -67,3 +88,7 @@ class Solution:
         # Space: O(n)
         return D.next
 ```
+
+## Explanation
+
+<YouTube id="q5a5OiGbT6Q" title="Merge K Sorted Lists - Leetcode 23 - Python" />