/**

  * Definition for a binary tree node.

  * struct TreeNode {

  *     int val;

  *     TreeNode *left;

  *     TreeNode *right;

  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

  * };

  */

 class Solution {

 private:

     vector<vector<int>> result;

 public:

     vector<vector<int>> levelOrder(TreeNode* root) {

         if (root == nullptr) {

             return result;

         }

         queue<TreeNode* > que;

         que.push(root);

         while (!que.empty()) {

             int sz = que.size();

             vector<int> temp;

             for (int i = ; i < sz; ++i) {

                 TreeNode* cur = que.front();

                 que.pop();

                 temp.push_back(cur -> val);

                 if (cur -> left != nullptr) {

                     que.push(cur -> left);

                 }

                 if (cur -> right != nullptr) {

                     que.push(cur -> right);

                 }

             }

             result.push_back(temp);

         }

         return result;

     }

 };