148. Sort List

这道题要求给链表排序，时间为O(nlogn)，并且使用的空间为常量。

根据时间复杂度，符合要求有快速排序，归并排序，堆排序。这里用归并排序：

首先用快慢指针把链表分成前后两段，并且把这两段切开，递归，不能分割了就merge

/**
* 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 || !head->next)    return head;
ListNode* slow = head, *fast = head, *pre = head;
while(fast && fast->next){
pre = slow;
fast = fast->next->next;
slow = slow->next;
}
pre->next = NULL;
return merge(sortList(head), sortList(slow));
}
ListNode* merge(ListNode* l1, ListNode* l2){
ListNode* dummy = new ListNode(-1);
ListNode* cur = dummy;
while(l1 && l2){
if(l1->val < l2->val){
cur->next = l1;
l1 = l1->next;
}else{
cur->next = l2;
l2 = l2->next;
}
cur = cur->next;
}
if(l1) cur->next = l1;
if(l2)  cur->next = l2;
return dummy->next;
}
};