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