Leetcode 101. Symmetric Tree

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

/** * 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)            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 (NULL == left && NULL == right)                continue;            if (NULL == left || NULL == right)                return false;            if (left->val != right->val)                return false;            q1.push(left->left);            q1.push(left->right);            q2.push(right->right);            q2.push(right->left);        }        return true;    }};