BM7 链表中环的入口结点

https://www.nowcoder.com/practice/253d2c59ec3e4bc68da16833f79a38e4?tpId=295&tqId=23449&ru=%2Fpractice%2Fff05d44dfdb04e1d83bdbdab320efbcb&qru=%2Fta%2Fformat-top101%2Fquestion-ranking&sourceUrl=%2Fexam%2Foj%3Fpage%3D1%26tab%3D%25E7%25AE%2597%25E6%25B3%2595%25E7%25AF%2587%26topicId%3D295

方案一： 快慢指针

GO

package main

func EntryNodeOfLoop(pHead *ListNode) *ListNode{
if pHead == nil || pHead.Next == nil{
return nil
}
fast := pHead
slow := pHead
for fast != nil && fast.Next != nil{
fast = fast.Next.Next
slow = slow.Next
if fast == slow{
//slow从pHead开始走，fast一次一个node的访问
slow = pHead
for fast != slow{
slow = slow.Next
fast = fast.Next
}
return fast
}
}
return nil

}
运行时间 6ms
 
占用内存 1916KB
 

方案二： 用哈希存储

package main

func EntryNodeOfLoop(pHead *ListNode) *ListNode{
if pHead == nil || pHead.Next == nil{
return nil
}
isVisit := make(map[*ListNode]bool)
pc := pHead
for pc != nil{
if isVisit[pc]{
return pc
}
isVisit[pc] = true
pc = pc.Next
}
return nil

}
运行时间 8ms
 
占用内存 2620KB
 

C++


/*
struct ListNode {
int val;
struct ListNode *next;
ListNode(int x) :
val(x), next(NULL) {
}
};
*/
class Solution {
public:
ListNode* EntryNodeOfLoop(ListNode* pHead) {
if(pHead == nullptr || pHead->next == nullptr){
return nullptr;
}
ListNode* fast = pHead;
ListNode* slow = pHead;

while(fast != nullptr && fast->next != nullptr){
fast = fast->next->next;
slow = slow->next;
if(fast == slow){
slow = pHead;
while(slow != fast){
slow = slow->next;
fast = fast->next;
}
return fast;
}
 
}
return nullptr;

}
};
运行时间 6ms
 
占用内存 776KB
 
posted @ 2023-02-01 20:06  重新出发123  阅读(33)  评论(0)    收藏  举报