Tree

LeetCode 102 note: BFS one whole level at a time, building the next level from the current one; an empty tree returns an empty list

LeetCode 1022 note: on the way down double the current number and add the node’s bit, adding it to the total at a leaf

LeetCode 993 note: cousins share a depth but not a parent; record x’s and y’s depth and parent during DFS and compare

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 703 note: keep a min-heap of size k; its top is the k-th largest

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