isValidBST.js

// https://leetcode.com/problems/validate-binary-search-tree/
// Javascript version for Validate BST Tree using iteration
var isValidBST = function(root) {
 if (!root) return true; 
 
 const stack = [];
 let prevNode = null;
 
 while(root != null || stack.length > 0) {
   while(root != null) {
     stack.push(root);
     root = root.left;
   }

   root = stack.pop();
   if (prevNode && prevNode.val >= root.val) return false;
   prevNode = root;
   root = root.right;
 }
 
 return true;
};