Consider all the leaves of a binary tree. From left to right order, the values of those leaves form a leaf value sequence.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Given the root node of a binary search tree (BST) and a value. You need to find the node in the BST that the node's value equals the given value. Return the subtree rooted with that node. If such node doesn't exist, you should return NULL.

Given an n-ary tree, return the postorder traversal of its nodes' values.

Given an n-ary tree, return the preorder traversal of its nodes' values.

For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees.

A full binary tree is a binary tree where each node has exactly 0 or 2 children.