1
var isValidBST = function (root) {
2
  return validate(root, -Infinity, Infinity);
3
};
4

5
var validate = function (node, lower, upper) {
6
  if (node == null) {
7
    // empty node or empty tree
8
    return true;
9
  }
10

11
  if (lower < node.val && node.val < upper) {
12
    // check if all tree nodes follow BST rule
13
    return (
14
      validate(node.left, lower, node.val) &&
15
      validate(node.right, node.val, upper)
16
    );
17
  } else {
18
    // early reject when we find violation
19
    return false;
20
  }
21
};