Validate Binary Search Tree

Description

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a 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:

    2
   / \
  1   3
Input: [2,1,3]
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Input: [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.

Solution(javascript)

/*
 * @lc app=leetcode id=98 lang=javascript
 *
 * [98] Validate Binary Search Tree
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
const isValidBST = function (root) {
  const aux = (node) => {
    if (!node) {
      return [true, null, null]
    }
    const [leftValid, leftMin, leftMax] = aux(node.left)
    const [rightValid, rightMin, rightMax] = aux(node.right)

    let valid = leftValid && rightValid
    if (leftMax !== null && leftMax >= node.val) {
      valid = false
    }
    if (rightMin !== null && rightMin <= node.val) {
      valid = false
    }
    const currentMin = leftMin === null ? node.val : leftMin
    const currentMax = rightMax === null ? node.val : rightMax
    return [valid, currentMin, currentMax]
  }
  return aux(root)[0]
}
// @lc code=end