【leetcode】148. Sort List

题目说明

https://leetcode-cn.com/problems/sort-list/description/
在 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。

解法1

使用归并排序对链表进行排序

/*
* 时间复杂度：O(nlogn)
* 归并排序的递归实现
/
ListNode sortList(ListNode* head) {
if (head == NULL || head->next == NULL)
return head;
ListNode *slow = head;
ListNode *fast = head->next;

while(fast && fast->next){
fast = fast->next->next;
slow = slow->next;
}

ListNode *half = slow->next;
slow->next = NULL;

return merge(sortList(head),sortList(half));
}

ListNode merge(ListNode head1,ListNode* head2)
{
ListNode *dummy = new ListNode(0);
ListNode *l3 = dummy;

ListNode *l1 = head1;
ListNode *l2 = head2;

while(l1 && l2 ){
if (l1->val < l2->val){
l3->next = l1;
l1 = l1->next;
} else if (l1->val >= l2->val){
l3->next = l2;
l2 = l2->next;
}
l3 = l3->next;
}
ListNode *last = (l1 == NULL)?l2:l1;
l3->next = last;

ListNode *ret = dummy->next;
delete dummy;
return ret;
}