98. Validate Binary Search Tree

Problem

Tags: Tree, Depth-First Search, Binary Search Tree, Binary Tree

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: 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, 10^4].
• -2^31 <= Node.val <= 2^31 - 1

Code

JS

// 98. Validate Binary Search Tree (6/8/53748)
// Runtime: 115 ms (18.45%) Memory: 43.55 MB (94.97%) 

/**
 * 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}
 */
function isValidBST(root, max = Infinity, min = -Infinity) {
    if (!root) return true;
    if (root.val >= max) return false;
    if (root.val <= min) return false;
    return isValidBST(root.left, root.val, min) && isValidBST(root.right, max, root.val);
}