Depth-First-Search

LeetCode 965 note: every value must equal the root’s; compare values while recursing, not the booleans the recursion returns

LeetCode 938 note: prune with the BST order, going only right below low and only left above high, adding nodes inside the range

LeetCode 897 note: in-order traversal appending nodes to a right-only chain with left children cleared; keep the dummy separate from the moving tail

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

LeetCode 783 note: a BST’s in-order is ascending, so the minimum difference is between in-order neighbors; remember the previous value and compare

LeetCode 733 note: DFS or BFS from the start, recoloring 4-directional cells that share the start’s original color; return immediately if the colors are equal

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 653 note: traverse while keeping a set of seen values; finding k - val in it means a pair exists

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