Merge k Sorted Lists [LeetCode]

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Summary:  Finds the smallest node every round, then links it to the one sorted list.

     ListNode *mergeKLists(vector<ListNode *> &lists) {
         ListNode * head = NULL;
         ListNode * pre_node = NULL;
         while(true){
             //find the smallest one
             ListNode * smallest_node = NULL;
             for(auto item : lists) {
                 if(item != NULL){
                     if(smallest_node == NULL || item->val < smallest_node->val )
                         smallest_node = item;
                 }
             }
             if(smallest_node == NULL)
                 break;

             if(pre_node == NULL){
                 pre_node = smallest_node;
                 head = pre_node;
             }else{
                 pre_node -> next = smallest_node;
                 pre_node = pre_node -> next;
             }

             for(int i = ; i< lists.size(); i ++){
                 if(lists[i] == smallest_node)
                     lists[i] = lists[i] -> next;
             }
         }
         return head;
     }