题目描述如下图所示：

解题思路：利用栈的先进后出特性去做；也可以利用队列去做，只要保证right先进队列就行

import java.util.*;
/**
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 tn = stack.pop();
           if(tn.left != null||tn.right != null){
                TreeNode temp = tn.left;
                tn.left = tn.right;
                tn.right = temp;
            }
            if(tn.left != null) {
                stack.push(tn.left);
            } 
            if(tn.right != null) {
                stack.push(tn.right);
            } 
        }
    }
}