C++刷LeetCode（148. 排序链表）【归并排序】
题目描述：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。
示例 1:
输入: 4->2->1->3
输出: 1->2->3->4
示例 2:
输入: -1->5->3->4->0
输出: -1->0->3->4->5
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/sort-list
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/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* merge(ListNode* head1, ListNode* head2){//递归
if(head1 == NULL)return head2;
if(head2 == NULL)return head1;
if(head1 -> val < head2 -> val){
head1 -> next = merge(head1 -> next, head2);
return head1;
}else{
head2 -> next = merge(head1, head2 -> next);
return head2;
}
}
ListNode* sortList(ListNode* head) {//递归
if(head == NULL || head -> next == NULL)return head;
ListNode* pre = head, *slow = head, *fast = head;
while(fast && fast -> next){
pre = slow;
slow = slow -> next;
fast = fast -> next -> next;
}//这样切出的slow会多出一个，因此要用pre
pre -> next = NULL;
return merge(sortList(head), sortList(slow));
}
};
参考链接：https://leetcode-cn.com/problems/sort-list/solution/lioney-cgui-bing-pai-xu-lian-biao-by-lioney/
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