Depth-First-Search

LeetCode 1372 note: each node returns two values, the longest zigzag starting left and starting right; going left extends the left child’s start-right value

LeetCode 124 note: DFS returns the best downward chain and updates the answer with left chain + node + right chain at every node; drop negative chains

LeetCode 314 note: BFS with a column index; BFS is what guarantees top-to-bottom within a column and left-to-right within a row

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 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 988 note: DFS prepending each character so the string at a leaf already reads leaf to root, and keep the smallest

LeetCode 236 note: recursion returns whichever of p or q a subtree contains; finding something on both sides means they split here, so this node is the LCA

LeetCode 2331 note: leaves are 0 or 1 and internal nodes are 2 for OR and 3 for AND, evaluated in post-order

LeetCode 129 note: on the way down multiply the current number by 10 and add the node’s value, adding it to the total at a leaf

LeetCode 117 note: connect next pointers level by level with BFS; for a non-perfect tree, looking only at the neighbour’s children breaks the chain

LeetCode 116 note: in a perfect binary tree, walk each level through the next pointers already set on the level above: left.next = right and right.next = next.left, with O(1) extra space

LeetCode 113 note: backtrack with the current path and store a copy at a leaf whose total matches; values can be negative, so no early pruning