trimBST = function (root, low, high) {

	if (!root) {
		return null;
	}

	if (root.val < low) {
		return trimBST(root.right, low, high);
	}

	if (root.val > high) {
		return trimBST(root.left, low, high);
	}

	root.left  = trimBST(root.left, low, high);
	root.right = trimBST(root.right, low, high);

	return root;
}
@param {TreeNode} root
@param {number} low
@param {number} high
@return {TreeNode}