Tree

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 590 note: post-order is each child then the node; iteratively, run a root-then-children-right-to-left pre-order and reverse the whole result

LeetCode 589 note: n-ary pre-order records the node then each child in order; flatten recursive results with extend, not by appending a list

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 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