Leetcode 98
https://leetcode.com/problems/validate-binary-search-tree/description/
Given the root of a binary tree, determine if it is a valid binary search tree (BST).
A valid BST is defined as follows:
The left subtree of a node contains only nodes with keys strictly less than the node's key. The right subtree of a node contains only nodes with keys strictly greater than the node's key. Both the left and right subtrees must also be binary search trees.
Example 1:
Input: root = [2,1,3] Output: true Example 2:
Input: root = [5,1,4,null,null,3,6] Output: false Explanation: The root node's value is 5 but its right child's value is 4.
Constraints:
The number of nodes in the tree is in the range [1, 104]. -231 <= Node.val <= 231 - 1