98.js

/** ------------------------------------------------------------------------------
 * 
 * 98. Validate Binary Search Tree
 * Topics: DFS, Binary Tree
 * https://leetcode.com/problems/validate-binary-search-tree/?envType=problem-list-v2&envId=depth-first-search
 * 
 ------------------------------------------------------------------------------ */
/**
 * 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) {
//   const dfs = (node) => {
//     if (!node.val) return true
//     if (!node.left && !node.right) return true
//     else if(node.right) {
//         if (node.val >= node.right.val) return false
//     } else {
//         if (node.val <= node.left.val) return false
//     }

//     return dfs(node.left) && dfs(node.right)
//   }

//   return dfs(root)
// }

var isValidBST = function (root, min = -Infinity, max = Infinity) {
  if (!root) return true
  if (root.val <= min || root.val >= max) return false

  return isValidBST(root.left, min, root.val) && isValidBST(root.right, root.val, max)
}

console.log(isValidBST(root))