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 less than the node's key.
- The right subtree of a node contains only nodes with keys greater than the node's key.
- Both the left and right subtrees must also be binary search trees.
Example 1:
![](https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg)
Input: root = [2,1,3] Output: true
Example 2:
![](https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg)
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
Solution(javascript)
/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @return {boolean}
*/
var isValidBST = function(root) {
return validate(root, low = null, high = null);
function validate(root, low, high) {
if (root == null) return true;
if ((low !== null && root.val <= low) ||
(high !== null && root.val >= high)) return false;
return validate(root.left, low, root.val) && validate(root.right, root.val, high);
}
};