Depth-First-Search

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 559 note: depth is 1 plus the deepest child, or 1 with no children

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 530 note: a BST’s in-order is ascending, so the minimum difference is between in-order neighbors; remember the previous value and compare

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 463 note: each land cell contributes 4 edges, minus 2 for each land neighbor above or to the left

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

LeetCode 257 note: DFS carrying the path string down, adding the whole path at each leaf

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

LeetCode 145 note: iterative post-order pushes a node back with a visited flag on its first pop, then the right and left children; record it on the second pop