class Solution {

 public:

     vector<vector<int> > Print(TreeNode* pRoot) {

         vector<vector<int>>res;

         BFS(pRoot, res);

         return res;

     }

     void BFS(TreeNode *root, vector<vector<int>>&res)

     {

         if (root == nullptr)return;

         queue<TreeNode*>q;

         q.push(root);

         bool fromLeft = true;

         while (!q.empty())

         {

             queue<TreeNode*>temp;

             vector<int>v;

             while (!q.empty())

             {

                 TreeNode* p = q.front();

                 q.pop();

                 v.push_back(p->val);

                 if (p->left != nullptr)temp.push(p->left);

                 if (p->right != nullptr)temp.push(p->right);

             }

             if(fromLeft)

                 res.push_back(v);

             else

             {

                 reverse(v.begin(), v.end());

                 res.push_back(v);

             }

             fromLeft = !fromLeft;

             q = temp;

         }

     }

 };