226.翻转二叉树

Snake8859 二叉树

# 题目描述

给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

示例 1：

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

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示例 2：

输入：root = [2,1,3]
输出：[2,3,1]

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示例 3：

输入：root = []
输出：[]

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# 外地解法-二叉树前序遍历

/**
 * 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 {
    public TreeNode invertTree(TreeNode root) {
        traverse(root);
        return root;
    }

    public void traverse(TreeNode node){
        if(node == null){
            return;
        }
        // 前序遍历 -> 交换左右
        TreeNode temp = node.left;
        node.left = node.right;
        node.right = temp;
        traverse(node.left);
        traverse(node.right);

    }
}

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# 外地解法-二叉树递归分解

/**
 * 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 {
    // 定义：将左右子树翻转,返回根节点
    public TreeNode invertTree(TreeNode root) {
        if(root == null){
            return null;
        }
        // 翻转左子树
        TreeNode left = invertTree(root.left);
        // 翻转右子树
        TreeNode right = invertTree(root.right);

        // 左右子树交换
        root.left = right;
        root.right = left;

        return root;
    }
}

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如何翻转二叉树？其实就是把二叉树上的每个节点的左右子节点都交换一下。