LeetCode 101. Symmetric Tree（镜像树）

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / 
  2   2
      
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
       if (!root) return true; 
        
        TreeNode *left;
        TreeNode *right;
        
        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;
    }
};