function TreeNode(val) {
  this.val = val;
  this.left = this.right = null;
}

/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function (root) {
  var helper = function (root, min, max) {
    if (root == null) return true;
    if (min != null && root.val <= min) return false
    if (max != null && root.val >= max) return false;
    return helper(root.left, min, root.val) && helper(root.right, root.val, max);
  }
  
  return helper(root, null, null);
};



var right = new TreeNode(15);
right.left = new TreeNode(6);
right.right = new TreeNode(20);
var root = new TreeNode(10);
root.left = new TreeNode(5);
root.right = right;

console.log(isValidBST(root));