problem:
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
Hide Tags Linked List Two Pointers
thinking:
(1)如果可以開設額外的空間,使用unordered_set存儲周遊過的結點,出現重複時即為存在環形結構
(2)如果不适用額外的空間,及空間複雜度為O(1),這裡使用快、慢雙指針。慢指針每次走一步,快指針每次走兩步。
如果存在環形結構,兩個指針總會相遇。
(3)終止條件也要注意:
fast!=NULL && fast->next!=NULL
防止出現fast->NULL->next
code:
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
ListNode *fast,*slow;
if(head==NULL) return false;
slow=head;
fast=head->next;
while(fast!=NULL && fast->next!=NULL)
{
if(slow==fast) return true;
slow=slow->next;
fast=fast->next->next;
}
return false;
}
};