Tree

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

LeetCode 107 note: a normal level-order traversal reversed at the end gives bottom-up order

LeetCode 103 note: a normal level-order traversal with every odd level reversed gives the zigzag

LeetCode 1379 note: walk both trees in lockstep; when the original reaches target, the clone’s node at the same position is the answer, comparing node identity

LeetCode 337 note: tree DP where each node returns the best (rob it, skip it) pair; not robbing alternate levels

LeetCode 106 note: the last post-order value is the root; taking roots from the end of the post-order, build the right subtree before the left

LeetCode 98 note: every node has a valid range (low, high); going left the upper bound becomes the node and going right the lower bound does; comparing parent and child is not enough

LeetCode 230 note: a BST’s in-order traversal is ascending, so stop an iterative in-order walk at the k-th node

LeetCode 199 note: in a level-order traversal, each level’s rightmost node is what you see from the right

LeetCode 1448 note: pass the path maximum down during DFS; a node is good when its value is at least that maximum

LeetCode 105 note: the first pre-order value is the root, and its position in the in-order tells the left subtree’s size; look positions up in a dictionary for O(n)