/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {

        return merge(lists, 0, lists.size()-1);
    }


    ListNode* merge(vector<ListNode*>& lists, int i, int j) {

        if(i == j) return lists[i];
        if(i+1 == j)
        {
            return merge2(lists[i], lists[j]);
        }
        int m = (i+j)/2;

        ListNode* a = merge(lists, i, m);
        ListNode* b = merge(lists, m+1, j);

    }

    ListNode* merge2(ListNode* l1, ListNode* l2){

        ListNode* pre = NULL, *tmp, *head;

        while(l1 != NULL && l2 != NULL)
        {
            if(l1->val > l2->val)
            {
                if(pre != NULL)
                {
                    tmp = l2;
                    l2 = l2->next;

                    tmp->next = l1;
                    pre->next = tmp;
                }
                else
                {
                    tmp = l2;
                    l2 = l2->next;

                    head = tmp;
                    pre = tmp;
                    pre->next = l1;
                }
            }
            else
            {
                if(head == NULL)
                {
                    head = l1;
                }
                pre = l1;
                l1 = l1->next;
            }
        }


        if(l2 != NULL)
        {
            l1->next = l2;
        }

        return head;
    }
};