Leetcode 156 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  

Solution 1: like linked list reverse.

 public class Solution {  
   public TreeNode upsideDownBinaryTree(TreeNode root) {  
     TreeNode left=null, p=null, x=root;  
     while (x!=null) {  
       TreeNode nextX=x.left;  
       TreeNode nextL=x.right;  
       x.right=p;  
       x.left=left;  
       p=x;  
       x=nextX;  
       left=nextL;  
     }  
     return p;  
   }  
 }