156.上下翻转二叉树

给你一个二叉树的根节点 root ，请你将此二叉树上下翻转，并返回新的根节点。

你可以按下面的步骤翻转一棵二叉树：

1. 原来的左子节点变成新的根节点
2. 原来的根节点变成新的右子节点
3. 原来的右子节点变成新的左子节点

 

 

 

 

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

    public TreeNode reverse(TreeNode curr,TreeNode next){
        // 递归出口：左下角        
        if(next == null){
            curr.right = parent;
            curr.left = right;
            return curr;
        }
        // 记录未来的 父节点和右侧兄弟
        TreeNode nextParent = curr;
        TreeNode nextRight = curr.right;
        next = curr.left;
        // 修改当前节点，进行翻转
        curr.left = right;
        curr.right = parent;
        parent = nextParent;
        right = nextRight;
        curr = next;
        next = curr.left;
        // 向下递归
        return reverse(curr,next);
    }
}