递归--上下翻转二叉树

https://leetcode-cn.com/problems/binary-tree-upside-down/

1 一个节点 返回原节点

2 两个节点 左变上，上变右

3 三个节点，右变左，左变上，上变右

4 >3 左子树已被翻转，则找到左子树最右节点，右变左，上变右

递归 java 

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