面试题26：合并k个排好序的单链表

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

 class Solution {
 public:
     ListNode *mergeKLists(vector<ListNode *> &lists) {
         int n = lists.size();
         if(n == ) return nullptr;
         if(n == ) return lists[];
         return mergeKLists(lists,,n-);
     }
     ListNode* mergeKLists(vector<ListNode*> &lists,int l,int r){
         if(l > r) return nullptr;
         if(l == r) return lists[l];

         int mid = l + (r-l)/;
         ListNode* left = mergeKLists(lists,l,mid);
         ListNode* right = mergeKLists(lists,mid+,r);

         return merge(left,right);
     }

     ListNode* merge(ListNode* l1,ListNode* l2){
         ListNode* dummy = new ListNode(-);
         ListNode* p = dummy;
         while(l1 && l2){
             if(l1->val <= l2->val){
                 p->next = l1;
                 l1 = l1->next;
                 p = p->next;
             }else{
                 p->next = l2;
                 l2 = l2->next;
                 p = p->next;
             }
         }
         if(l1) p->next = l1;
         if(l2) p->next = l2;
         return dummy->next;

     }
 };