148. Sort List 链表归并排序

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
void mergeLink(ListNode* head,ListNode* mid, ListNode* last){
ListNode* head1=head;
ListNode* end1 = mid;
ListNode* rehead2 = mid->next;//记录第一个链表的结尾
ListNode* head2 = mid->next,*end2 = last;
ListNode* temp1,*temp2;
while(head1->next!=rehead2){
while(head1->next!=rehead2 && head1->next->val<=head2->val){//当第一个链表小雨第二个链表就一直遍历
head1=head1->next;
}
if(head1->next == rehead2){//当第一个链表到尾了直接把第二个链表接到第一个链表上
head1->next = head2;
break;
}else{
temp2 = head2->next;
temp1 = head1->next;
head1->next = head2;
head2->next = temp1;
head2 = temp2;
head1=head1->next;
if(head2 == end2){//当第二个链表到尾了，找到第一个链表的尾然后把第二个链表的尾接到第一个链表的尾
while(head1->next!=rehead2) head1 = head1->next;
head1->next = head2;
break;
}
}
}
}
void mergesort(ListNode* head,ListNode* last){
ListNode* fast = head,*slow = head;
if(head->next == last || head->next->next==last) return;
while(fast!=last && fast->next!=last){//寻找中位数
fast=fast->next->next;
slow = slow->next;
}
mergesort(head,slow->next);
mergesort(slow,last);
mergeLink(head,slow,last);//合并两个链表
return;
}
class Solution {
public:
ListNode* sortList(ListNode* head) {
if(head==NULL || head->next == NULL) return head;
ListNode* newHead = new ListNode(0);
newHead->next = head;
mergesort(newHead,nullptr);
return newHead->next;
}
};