var helper = function (root) {
  if (!root.left && !root.right) {
    return [root, root];
  }
  if (!root.left) {
    return [root, helper(root.right)[1]]
  }
  if (!root.right) {
    var temp = root.left;
    root.right = helper(temp)[0];
    root.left = null;
    return [root, helper(temp)[1]]
  }
  var temp = root.right;
  root.right = helper(root.left)[0];
  helper(root.left)[1].right = temp;
  root.left = null;
  return [root, helper(root.right)[1]]
}
var flatten = function (root) {
  if (!root) return null;
  helper(root);
};