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 = 0; i< lists.size(); i ++){
                if(lists[i] == smallest_node)
                    lists[i] = lists[i] -> next;
            }
        }
        return head;
    }