1
var trimBST = function (root, low, high) {
2
  if (!root) return null;
3
  if (root.val < low) root = trimBST(root.right, low, high);
4
  else if (root.val > high) root = trimBST(root.left, low, high);
5
  else {
6
    root.left = trimBST(root.left, low, high);
7
    root.right = trimBST(root.right, low, high);
8
  }
9
  return root;
10
};