Medium

LeetCode 109 note: take the middle found with fast and slow pointers as the root and recurse on both halves; the height balances naturally

LeetCode 236 note: recursion returns whichever of p or q a subtree contains; finding something on both sides means they split here, so this node is the LCA

LeetCode 222 note: in a complete tree, equal leftmost depths on both sides mean one side is perfect, counted as 2^h - 1, for O(log² n)

LeetCode 129 note: on the way down multiply the current number by 10 and add the node’s value, adding it to the total at a leaf

LeetCode 117 note: connect next pointers level by level with BFS; for a non-perfect tree, looking only at the neighbour’s children breaks the chain

LeetCode 116 note: in a perfect binary tree, walk each level through the next pointers already set on the level above: left.next = right and right.next = next.left, with O(1) extra space

LeetCode 113 note: backtrack with the current path and store a copy at a leaf whose total matches; values can be negative, so no early pruning

LeetCode 107 note: a normal level-order traversal reversed at the end gives bottom-up order

LeetCode 103 note: a normal level-order traversal with every odd level reversed gives the zigzag

LeetCode 402 note: a monotonically increasing stack; while deletions remain, pop any top larger than the current digit, then strip leading zeros

LeetCode 334 note: keep the smallest first and the smallest second that has something smaller before it; any number above second completes a triplet

LeetCode 324 note: sort, split into a small half and a large half, reverse each, and interleave with small values at even indices and large ones at odd indices