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.

Example

Example1

Input: {1,2,3,4,5}
Output: {4,5,2,#,#,3,1}
Explanation:
The input is
    1
   / \
  2   3
 / \
4   5
and the output is
   4
  / \
 5   2
    / \
   3   1

Example2

Input: {1,2,3,4}
Output: {4,#,2,3,1}
Explanation:
The input is
    1
   / \
  2   3
 /
4
and the output is
   4
    \
     2
    / \
   3   1

public class Solution {
    public TreeNode upsideDownBinaryTree(TreeNode root) {
        if (root == null)
            return null;
        if (root.left == null && root.right == null)
            return root;
        // At this point it is stated, that left is guaranteed to exist
        TreeNode ans = upsideDownBinaryTree(root.left);
        root.left.right = root;
        root.left.left = root.right;
        root.left = null;
        root.right = null;
        return ans;
    }
}