156. Binary Tree Upside Down

Medium

这道题思路不是很正常，就只能观察树的变化然后写出相应的code，没有什么通用性，记住并且能口述分析就可以了.

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

简单画了下recursion tree和变化过程方便理解

image.png