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Validate Binary Search Tree

Problem statement​

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:

Input: root = [2,1,3]Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]Output: falseExplanation: 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

My solution​

/**
 * 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) {
    function traverse(node, min, max) {
        if(!node) {
            return true;
        }
        
        if ((min !== null && node.val <= min) || (max !== null && node.val >= max)) {
            return false
        }
        
        return traverse(node.left, min, node.val) && traverse(node.right, node.val, max)
    }
    
    return traverse(root, null, null)
};