class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> v;
        if(root == NULL) return v;
        vector<int> vv;
        queue<TreeNode *> q;
        q.push(root);
        q.push(NULL);
        while(!q.empty()){
            if(q.front() == NULL){
                q.pop();
                v.push_back(vv);
                vv.clear();
                if(!q.empty()) q.push(NULL);
            }
            else if(q.front() != NULL){
                if(q.front() -> left != NULL) q.push(q.front() -> left);
                if(q.front() -> right != NULL)  q.push(q.front() -> right);
                vv.push_back(q.front() -> val);
                q.pop();
            }
            
        }
        return v;
    }
};