Use merge two looply merge the from the queue.

1 /** 2  * Definition for singly-linked list. 3  * struct ListNode { 4  *     int val; 5  *     ListNode *next; 6  *     ListNode(int x) : val(x), next(NULL) {} 7  * }; 8  */ 9 class Solution {10 public:11     ListNode *merge(ListNode *l1, ListNode *l2) {12         if (!l1) return l2;13         if (!l2) return l1;14         ListNode *result;15         if (l1->val < l2->val) {16             result = l1;17             result->next = merge(l1->next, l2);18         } else {19             result = l2;20             result->next = merge(l1, l2->next);21         }22         return result;23     }24     ListNode *mergeKLists(vector
     
       &lists) {25         int len = lists.size(), current = len;26         if (len == 0) return NULL;27         queue
      
        q;28         for (int i = 0; i < len; i++) {29             q.push(lists[i]);30         }31         while (current > 1) {32             ListNode *l1 = q.front();33             q.pop();34             ListNode *l2 = q.front();35             q.pop();36             q.push(merge(l1, l2));37             current--;38         }39         return q.front();40     }41 };