Tree

LeetCode 987 note: collect (col, row, val) triples, sort them, and group by column

LeetCode 2973 note: the max product of three is either the three largest or the two smallest times the largest, so each subtree returns only its 2 smallest and 3 largest values, merged in post-order

LeetCode 1028 note: a stack holds the path from the root to the current node, so its length is the depth

LeetCode 2509 note: in heap numbering the parent is v // 2, so climb the larger node until both meet at the LCA

LeetCode 1373 note: a post-order traversal returns (sum, min, max, is_bst); a node roots a BST when the left max < its value < the right min

LeetCode 1377 note: undirected edges, seen must hold nodes, and an early arrival depends on whether the frog can still move

LeetCode 2421 note: add nodes in increasing value order and count same-value nodes per Union-Find group

LeetCode 2603 note: trim coin-free leaves, trim exactly two more layers, and walk each remaining edge twice

LeetCode 2196 note: keep nodes in a dict and link them in one pass; the root is the only value that is nobody’s child

LeetCode 3820 note: three BFS runs give every node’s distance to the three targets

LeetCode 865 note: BFS to the last level and climb together, or one DFS returning (depth, node)

LeetCode 1339 note: compute the total, enumerate every subtree sum, and leave the input untouched