Binary-Tree

LeetCode 872 note: collect leaves left to right with DFS; BFS collects level by level, which is not left-to-right order

LeetCode 700 note: go left when the node is larger, right when smaller, and return the node when found

LeetCode 671 note: the root is the minimum; the second minimum is the smallest value larger than the root, and only subtrees equal to the root need searching

LeetCode 637 note: level-order traversal, dividing each level’s sum by its count

LeetCode 606 note: output in pre-order, keeping an empty () when only the left subtree is missing, and omitting the right parentheses when it is empty

LeetCode 572 note: for every node of root, check whether its subtree equals subRoot; recurse with isSubtree, not by comparing only the children

LeetCode 563 note: return subtree sums in post-order; each node’s tilt is the difference of its subtree sums, added to the answer

LeetCode 543 note: the diameter may skip the root; return heights in post-order while updating the answer with left + right height at every node, counting edges

LeetCode 501 note: a BST’s in-order is sorted, so equal values are contiguous; count the current run during in-order, resetting the answer on a new maximum and appending on a tie

LeetCode 404 note: tell each child during DFS whether it is a left child, adding only left children that are leaves

LeetCode 235 note: go left when both p and q are smaller, right when both are larger; otherwise (split, or one is the node itself) this node is the LCA

LeetCode 226 note: swap every node’s children, using a simultaneous assignment so the already-changed side is not reused