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class Solution {
public:
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        if(lists.size() <= 0)
            return NULL;
        while(lists.size() > 1) {
            vector<ListNode *>lists_tmp;
            lists_tmp.clear();
            ListNode * tmp;
            int i = 0;
            while (true) {
                tmp = merge2Lists(lists[2*i],lists[2*i+1]);
                lists_tmp.push_back(tmp);
                i++;
                if(2*i+1 == lists.size()) {
                    lists_tmp.push_back(lists[2*i]);
                    break;
                }
                if(2*i+1 > lists.size()) {
                    break;
                }
            }
            lists.clear();
            lists = lists_tmp;
        }
        return lists[0];
    }
private:
    ListNode *merge2Lists(ListNode * list1,ListNode * list2) {
        ListNode * list = NULL;
        ListNode * list_c = NULL;
        while(list1 != NULL && list2 != NULL) {
            if(list1->val < list2->val) {
                if(list == NULL) {
                    list = list1;
                    list_c = list;
                }
                else {
                    list_c->next = list1;
                    list_c = list_c->next;
                }
                list1 = list1->next;
            }
            else {
                if(list == NULL) {
                    list = list2;
                    list_c = list;
                }
                else {
                    list_c->next = list2;
                    list_c = list_c->next;
                }
                list2 = list2->next;
            }
        }
        while(list1!=NULL) {
            if(list_c == NULL) {
                list_c = list1;
                list = list_c;
                list1 = list1->next;
                continue;
            }
            list_c->next = list1;
            list_c= list_c->next;
            list1 = list1->next;
        }
        while(list2!=NULL) {
            if(list_c == NULL) {
                list_c = list2;
                list = list_c;
                list2 = list2->next;
                continue;
            }
            list_c->next = list2;
            list_c= list_c->next;
            list2 = list2->next;
        }
        if(list_c != NULL)list_c->next = NULL;
        return list;
    }
};