Tree

LeetCode 701 note: walk down the BST to an empty spot and attach the new node; an empty tree must return the new node

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 429 note: BFS level order with the children list in place of left and right

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 3787 note: two BFS passes find one diameter’s ends A and B; every node’s farthest node is A or B, so a node is a diameter endpoint exactly when max(dist(A,i), dist(B,i)) equals the diameter

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 3203 note: connect the two centers; the answer is max(d1, d2, ceil(d1/2) + ceil(d2/2) + 1)

LeetCode 3313 note: the last marked node is the farthest one, and a farthest node can always be a diameter endpoint