LeetCode - Binary Tree Level Order Traversal

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if(!root) return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            vector<int> t;
            int s = q.size();
            for(int i=0;i<s;i++){
                TreeNode* tt = q.front(); q.pop();
                t.push_back(tt->val);
                if(tt->left) q.push(tt->left);
                if(tt->right) q.push(tt->right);
            }
            res.push_back(t);
        }
        return res;
    }
};