leetcode中symmetric tree 问题

##问题描述
>

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

>

But the following is not:

  1
 / 
2   2
    
 3    3

##C++ code

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        TreeNode *left,*right;
        if(root == NULL) return true;
        queue<TreeNode *> q1,q2;
        q1.push(root->left);
        q2.push(root->right);
        while(!q1.empty() && !q2.empty()){
            left = q1.front();
            q1.pop();
            right = q2.front();
            q2.pop();
            if(left == NULL && right == NULL){
                continue;
            }
            if(left == NULL || right == NULL){
                return false;
            }
            q1.push(left->left);
            q1.push(left->right);
            q2.push(right->right);
            q2.push(right->left);
        }
        return true;
    }
}

##问题分析

该问题要利用一个队列存储节点