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
