Leetcode Binary Tree Upside Down

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new
root.
For example:
Given a binary tree 

{1,2,3,4,5}

,

1
/ \
2   3
/ \
4   5

return the root of the binary tree 

[4,5,2,#,#,3,1]

.

4
/ \
5   2
/ \
3   1

Difficulty: Medium

Solution: From bottom to up, use recursion.

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
TreeNode ans = null;
public void helper(TreeNode root){
if(root == null)
return;
TreeNode leftNode = root.left;
TreeNode rightNode = root.right;
if(leftNode == null){
ans = root;
return;
}
helper(leftNode);
leftNode.left = rightNode;
leftNode.right = root;
root.left = null;
root.right = null;
}
public TreeNode upsideDownBinaryTree(TreeNode root) {
helper(root);
return ans;
}
}