104. merge k sorted lists

class  {
public:
    
     * @param lists: a list of ListNode
     * @return: The head of one sorted list.
     */
    ListNode *mergeKLists(vector<ListNode *> &lists) {
      // write your code here
     ListNode *root = NULL, *back_node = NULL;

     while (true)
       {
            int min = std::numeric_limits<int>::max();
         vector<ListNode *>::iterator min_iter = lists.end();
         for (auto iter = lists.begin(); iter != lists.end(); ++iter)
           {
                ListNode *cur_node = *iter;
              if (cur_node != NULL && cur_node->val < min)
             {
                    min = cur_node->val;
                  min_iter = iter;
             }
            }

           if (min_iter == lists.end())
            {
                break;
          }

           ListNode *min_node = *min_iter;
          if (root == NULL)
          {
                root = min_node;
             back_node = min_node;
            }
            else
            {
                back_node->next = min_node;
               back_node = min_node;
            }

           *min_iter = min_node->next;
           min_node->next = NULL;
       }

       return root;
    }
};