LeetCode() Symmetric Tree

/**
 * 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;
        queue<TreeNode*> q;
        q.push(root->left);
        q.push(root->right);
        while(!q.empty()){
            TreeNode* lt=q.front();
            q.pop();
            TreeNode* rt=q.front();
            q.pop();
            if(!rt&&!lt)    continue;
            if(!rt||!lt)    return false;
            if(lt->val!=rt->val)    return false;
            q.push(lt->left);
            q.push(rt->right);
            q.push(lt->right);
            q.push(rt->left);
        }
        return true;
    }
    bool isSymmetricHelper(TreeNode* l,TreeNode* r){
        if(!r&&!l)  return true;
        if((!r&&l)||(!l&&r)||(r->val!=l->val))  return false;
        return isSymmetricHelper(l->left,r->right)&&isSymmetricHelper(l->right,r->left);

    }
};