一、题目说明
题目148. Sort List,对链表进行排序,时间复杂度要求是O(nlog(n)),空间复杂度要求是常量。难度是Medium!
二、我的解答
根据要求,唯一符合标准的是归并排序。
class Solution{
public:
ListNode* sortList(ListNode* head){
if(head==NULL || head->next==NULL) return head;
return mergeSort(head);
}
//归并排序
ListNode* mergeSort(ListNode* node){
if(node==NULL || node->next==NULL) return node;
ListNode* fast= node,*slow = node;
ListNode* breakNode = node;//breakNode 指向l1的最后一个元素
//也可以采用先遍历一遍,统计链表节点的数量,然后归并排序
//找到链表中间
while(fast && fast->next){
fast = fast->next->next;
breakNode = slow;
slow = slow->next;
}
breakNode->next = NULL;
ListNode* l1 = mergeSort(node);
ListNode* l2 = mergeSort(slow);
return merge(l1,l2);
}
//合并两个链表 recursive
ListNode* merge(ListNode* l1,ListNode* l2){
if(l1 == NULL){
return l2;
}
if(l2 == NULL){
return l1;
}
if(l1->val < l2->val){
l1->next = merge(l1->next,l2);
return l1;
}else{
l2->next = merge(l2->next,l1);
return l2;
}
}
};
性能如下:
Runtime: 60 ms, faster than 41.32% of C++ online submissions for Sort List.
Memory Usage: 16.2 MB, less than 15.00% of C++ online submissions for Sort List.
三、优化措施
将merge函数,修改为非递归版本:
class Solution{
public:
ListNode* sortList(ListNode* head){
if(head==NULL || head->next==NULL) return head;
return mergeSort(head);
}
//归并排序
ListNode* mergeSort(ListNode* node){
if(node==NULL || node->next==NULL) return node;
ListNode* fast= node,*slow = node;
ListNode* breakNode = node;//breakNode 指向l1的最后一个元素
//找到链表中间
while(fast && fast->next){
fast = fast->next->next;
breakNode = slow;
slow = slow->next;
}
breakNode->next = NULL;
ListNode* l1 = mergeSort(node);
ListNode* l2 = mergeSort(slow);
return merge(l1,l2);
}
//合并两个链表
ListNode* merge(ListNode* l1,ListNode* l2){
if(l1 == NULL) return l2;
if(l2 == NULL) return l1;
ListNode dummy(0);
ListNode* p = &dummy;
while(l1!=NULL && l2!=NULL){
if(l1->val < l2->val){
p->next = l1;
l1 = l1->next;
}else{
p->next = l2;
l2 = l2->next;
}
p = p->next;
}
if(l1 !=NULL){
p->next = l1;
}
if(l2 !=NULL){
p->next = l2;
}
return dummy.next;
}
};
性能如下:
Runtime: 52 ms, faster than 67.44% of C++ online submissions for Sort List.
Memory Usage: 15 MB, less than 15.00% of C++ online submissions for Sort List.
原文地址:https://www.cnblogs.com/siweihz/p/12274559.html
时间: 2024-10-25 18:26:52