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

分析：

对于树，递归总是是很好的实现方法，所以这题应该

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

         root.left.right = root;
         root.left.left = root.right;

         root.left = null;
         root.right = null;

         return newRoot;
     }
 }