156. Binary Tree Upside Down

 1 class Solution {
 2     TreeNode newRoot;
 3     TreeNode prev;
 4     public TreeNode upsideDownBinaryTree(TreeNode root) {
 5         if(root == null) return root;
 6         helper(root);
 7         return newRoot;
 8     }
 9     
10     public void helper(TreeNode root){
11         if(root.left != null){
12             helper(root.left);
13         }else{
14             newRoot = new TreeNode(root.val);
15             prev = newRoot;
16             return;
17         }
18         if(root.right != null){
19             prev.left = new TreeNode(root.right.val);
20         } 
21         prev.right = new TreeNode(root.val);
22         prev = prev.right;
23     }
24 }