在使用O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。
详见：https://leetcode.com/problems/sort-list/description/
方法一：归并排序
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* sortList(ListNode* head) {
if(head==nullptr||head->next==nullptr)
{
return head;
}
ListNode* slow=head;
ListNode* fast=head;
ListNode* mid=nullptr;
while(fast&&fast->next)
{
mid=slow;
slow=slow->next;
fast=fast->next->next;
}
mid->next=nullptr;
return mergeHelper(sortList(head),sortList(slow));
}
ListNode* mergeHelper(ListNode* low,ListNode* high)
{
ListNode* helper=new ListNode(-1);
ListNode* cur=helper;
while(low&&high)
{
if(low->val<high->val)
{
cur->next=low;
low=low->next;
}
else
{
cur->next=high;
high=high->next;
}
cur=cur->next;
}
cur->next=low?low:high;
return helper->next;
}
};
方法二：快速排序
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* sortList(ListNode* head) {
if(head==nullptr||head->next==nullptr)
{
return head;
}
quickSort(head,nullptr);
return head;
}
void quickSort(ListNode* begin,ListNode* end)
{
if(begin!=end)
{
ListNode* sep=getSeperator(begin,end);
quickSort(begin,sep);
quickSort(sep->next,end);
}
}
ListNode* getSeperator(ListNode* begin,ListNode* end)
{
ListNode* first=begin;
ListNode* second=begin->next;
int key=begin->val;
while(second!=end)
{
if(second->val<key)
{
first=first->next;
swap(first,second);
}
second=second->next;
}
swap(first,begin);
return first;
}
void swap(ListNode* a,ListNode* b)
{
int tmp=a->val;
a->val=b->val;
b->val=tmp;
}
};