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

/**
 * 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 root;
        if(root.left == null && root.right == null) return root;

        TreeNode newRoot = upsideDownBinaryTree(root.left);
        TreeNode newLeft = root.right;
        TreeNode oldLeft = root.left;

        root.left = null;
        root.right = null;
        oldLeft.left = newLeft;
        oldLeft.right = root;

        return newRoot;
    }
}