LC102 Binary level traversal

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],
The result will be
[
[3],
[9,20],
[15,7]
]

    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> q;
        queue<int> level;
        q.push(root);
        level.push(1);
        int levelCount = 1;
        vector<vector<int>> result;
        vector<int> res;

        while(!q.empty()){
            int curLevel = level.front();
            level.pop();
            TreeNode* curNode = q.front();
            q.pop();
            if(curLevel != levelCount){
                levelCount = curLevel;
                result.push_back(res);
                res.clear();
            }
            if(curNode != NULL){
                res.push_back(curNode -> val);
                q.push(curNode -> left);
                q.push(curNode -> right);
                level.push(levelCount + 1);
                level.push(levelCount + 1);
            }
        }

        return result;
    }