class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        queue<TreeNode*> q;
        if(root)q.push(root);
        while(q.size())
        {
            vector<int> level;
            int len=q.size();
            while(len--)
            {
                auto a=q.front();
                q.pop();
                level.push_back(a->val);
                if(a->left)q.push(a->left);
                if(a->right)q.push(a->right);
            }
            res.push_back(level);
        }
        return res;
    }
};