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 

/**
 * 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) return null;
        TreeNode left = null, right = null;
        TreeNode nextRoot = root.left, nextLeft = root.right;
        while (root != null) {
            root.left = left;
            root.right = right;
            left = nextLeft;
            right = root;
            root = nextRoot;
            if (root == null) break;
            nextRoot = root.left;
            nextLeft = root.right;
        }
        return right;
    }
}