题目描述

操作给定的二叉树，将其变换为源二叉树的镜像。

https://www.nowcoder.com/practice/564f4c26aa584921bc75623e48ca3011

解题想法

递归思路：交换其左右子树，然后递归调用，交换子树。

非递归思路：涉及到反转，就应该想到栈，使用栈进行反转交换

代码实现

• 递归版本

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    public void Mirror(TreeNode root) {
        if (root == null){
            return;
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        Mirror(root.left);
        Mirror(root.right);
    }   
}

• 使用栈

import java.util.Stack;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    public void Mirror(TreeNode root) {
        if(root == null){
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode rootTemp = stack.pop();
            TreeNode temp1 = rootTemp.left;
            rootTemp.left = rootTemp.right;
            rootTemp.right = temp1;
            if (rootTemp.left != null){
                stack.push(rootTemp.left);
            }
            if (rootTemp.right != null){
                stack.push(rootTemp.right);
            }
        }
    }
}