/**
knakul853
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {

        if( lists.size() < 1) return NULL;

        while(lists.size() > 1)
        {
           ListNode* temp = merge(lists[0], lists[1]);
            lists.erase(lists.begin());
            lists.erase(lists.begin());
            lists.push_back(temp);
        }

        return lists[0];

    }

    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        if( !l1 ) return l2;
        if(!l2) return l1;

        if(l1->val <= l2->val)
        {
            l1->next = merge(l1->next, l2);
            return l1;
        }

        else{

            l2->next = merge(l1, l2->next);
            return l2;
        }
    }
};