翻转二叉树

一、题目

226. 翻转二叉树

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：

img

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

示例 2：

img

输入：root = [2,1,3]
输出：[2,3,1]

示例 3：

输入：root = []
输出：[]

提示：

  • 树中节点数目范围在 [0, 100] 内
  • -100 <= Node.val <= 100

二、题解

题解一（广度优先）

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public static TreeNode invertTreeFromBFS(TreeNode root) {
    if (root == null || (root.left == null && root.right == null)) {
        return root;
    }
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    while (!queue.isEmpty()) {
        TreeNode tmp = queue.poll();
        TreeNode left = tmp.left;
        tmp.left = tmp.right;
        tmp.right = left;
        if(tmp.left!=null) {
            queue.add(tmp.left);
        }
        if(tmp.right!=null) {
            queue.add(tmp.right);
        }
    }
    return root;
}

image-20241209200517428

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public static TreeNode invertTree(TreeNode root) {
    if (root == null) {
        return null;
    }
    TreeNode temp = root.left;
    root.left = root.right;
    root.right = temp;
    invertTree(root.left);
    invertTree(root.right);
    return root;
}

image-20241209200536621

三、总结

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package com.loltoulan.binary_tree;

import java.util.LinkedList;
import java.util.Queue;

public class InvertTree {

    public static void main(String[] args) {
        TreeNode root = TreeNode.createTree();
        StringBuilder sb = new StringBuilder();
        System.out.println(TreeNode.levelOrderTree(invertTree(root), sb));
    }

    public static TreeNode invertTreeFromBFS(TreeNode root) {
        if (root == null || (root.left == null && root.right == null)) {
            return root;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode tmp = queue.poll();
            TreeNode left = tmp.left;
            tmp.left = tmp.right;
            tmp.right = left;
            if(tmp.left!=null) {
                queue.add(tmp.left);
            }
            if(tmp.right!=null) {
                queue.add(tmp.right);
            }
        }
        return root;
    }

    public static TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}