Binary-Tree

LeetCode 687 note: DFS returns the longest same-value downward chain in edges, and each node updates the answer with left + right

LeetCode 515 note: BFS level order, taking the maximum of each level; start from -inf

LeetCode 173 note: flattening to an array is simple but uses O(n) space; the follow-up keeps the left spine on a stack for amortized O(1) and O(h) space

LeetCode 1372 note: each node returns two values, the longest zigzag starting left and starting right; going left extends the left child’s start-right value

LeetCode 124 note: DFS returns the best downward chain and updates the answer with left chain + node + right chain at every node; drop negative chains

LeetCode 314 note: BFS with a column index; BFS is what guarantees top-to-bottom within a column and left-to-right within a row

DFS return values, BFS level order, BST bounds, climbing to the LCA, tree diameter, and leaf trimming

LeetCode 987 note: collect (col, row, val) triples, sort them, and group by column

LeetCode 1028 note: a stack holds the path from the root to the current node, so its length is the depth

LeetCode 2509 note: in heap numbering the parent is v // 2, so climb the larger node until both meet at the LCA

LeetCode 2196 note: keep nodes in a dict and link them in one pass; the root is the only value that is nobody’s child

LeetCode 865 note: BFS to the last level and climb together, or one DFS returning (depth, node)