解法一：递归

public class Solution {
    public void Mirror(TreeNode root) {
        root=reverse(root);
    }

    TreeNode reverse(TreeNode root){
        if(root==null) return root;
        TreeNode t=root.left;
        root.left=reverse(root.right);
        root.right=reverse(t);
        return root;
    }
}

解法二：迭代

import java.util.*;
public class Solution {
    public void Mirror(TreeNode root) {
        if(root==null) return;
        LinkedList<TreeNode> q=new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()){
            TreeNode curr=q.poll();
            if(curr.left!=null) q.add(curr.left);
            if(curr.right!=null) q.add(curr.right);
            TreeNode t=curr.left;
            curr.left=curr.right;
            curr.right=t;
        }
    }
}