按之字形顺序打印二叉树

层次遍历打印二叉树，用队列实现。

vector<vector<int> > Print(TreeNode* pRoot) {
     vector<vector<int>> res;    //存放结果

     if(pRoot == NULL) return res;

     queue<TreeNode*> q;
     q.push(pRoot);

     int flag = 0;  //用于判断是否需要反转存储

     while(!q.empty()){
         int level_size = q.size();    //每层vector大小            
         vector<int> path; path.resize(level_size);   

         for(int i = 0; i<level_size;i++){
             TreeNode * tmp = q.front(); q.pop();

             if(flag == 0){//不反转
                 path[i] = tmp->val;                    
             }else{    //反转
                 path[level_size-1 - i] = tmp->val;                   
             }

             if(tmp->left) q.push(tmp->left);
             if(tmp->right) q.push(tmp->right);                
         }
         res.push_back(path);

         //每层结束后需要反转
         if(flag == 0){flag = 1;}
         else flag = 0;            
     }

     return res;
 }