【一天一道LeetCode】#23. Merge k Sorted Lists

一天一道LeetCode系列

（一）题目

（二）解题

/**

* 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) {

int len = lists.size();

if(len == 0) return NULL;

if(len == 1) return lists[0];

ListNode* ret =  merge(lists,0,lists.size()-1);

return ret;

}

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

if(i==j) return lists[i];

ListNode* ret = NULL;

if(i<j){

int mid = (i+j)/2;

ListNode* lhs = merge(lists , i,mid);

ListNode* rhs = merge(lists , mid+1,j);

ret = mergeTwoLists(lhs,rhs);

}

return ret;

}

ListNode* mergeTwoLists(ListNode* list1 , ListNode* list2){

if(list1 == NULL) return list2;

if(list2 == NULL) return list1;

ListNode* ret = NULL;

if(list1->val < list2->val){

ret = list1;

ret->next = mergeTwoLists(list1->next ,  list2);

}

else{

ret = list2;

ret->next = mergeTwoLists(list1 ,  list2->next);

}

return ret;

}

};