【LeetCode】23. Merge k Sorted Lists

合并k个已合并链表。

思路：先把链表两两合并，直到合并至只有一个链表

 /**
  * 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) {
         if(lists.empty()) return NULL;
         int len = lists.size();
         while(len>){
             for(int i=;i<len/;i++){
                 lists[i] = mergeTwoLists(lists[i],lists[len--i]);
             }
             len = (len+)/;   //这个技巧很有用
         }
         return lists.front();
     }
     ListNode* mergeTwoLists(ListNode*l1,ListNode*l2){
         if(NULL==l1) return l2;
         else if(NULL==l2) return l1;
         if(l1->val<l2->val){
             l1->next = mergeTwoLists(l1->next,l2);
             return l1;
         }
         else {
             l2->next = mergeTwoLists(l1,l2->next);
             return l2;
         }
     }
 };