Medium

LeetCode 99 note: find the inversions in the in-order sequence: the earlier node of the first inversion and the later node of the last one were swapped

LeetCode 450 note: once the node is found: with zero or one child, splice the child in; with two, copy the right subtree’s minimum and delete that node

LeetCode 449 note: for a BST, pre-order without null markers is enough; deserialize recursively with bounds in O(n)

LeetCode 331 note: count open slots: start with 1, every node fills one, a non-null node opens two more; never go negative and end at exactly 0

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 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