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class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root){ 
        vector<vector<int>> ans;
        queue<TreeNode*> q;
        if(root) q.push(root);

        while(!q.empty()){
            int len = q.size();
            vector<int> level;
            while(len --){
                auto t = q.front();
                q.pop();
                level.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            ans.push_back(level);
        }
        return ans;
    }
};