Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 bool hasCycle(ListNode *head) { 12 if(head==NULL) 13 return false; 14 ListNode *p=head,*q; 15 int n=0,m; 16 while(p!=NULL) 17 { 18 q=head; 19 m=0; 20 while(m<=n) 21 { 22 if(q==p->next) 23 return true; 24 else 25 { 26 q=q->next; 27 m++; 28 } 29 } 30 p=p->next; 31 n++; 32 } 33 return false; 34 } 35 };
后记:在做Linked List Cycle II 的时候,发现上述解法效率太低,所以换了种思路:
(1)定义slow和fast指针,都指向head结点,然后slow指针每次向后走1个结点,fast指针每次向后走2个结点。
(2)如果fast指针为空,则说明不存在环;如果fast指针和slow指针相等,则说明存在环。
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 bool hasCycle(ListNode *head) { 12 if(head==NULL||head->next==NULL) 13 return false; 14 ListNode *slow=head,*fast=head; 15 while(fast!=NULL) 16 { 17 slow=slow->next; 18 fast=fast->next; 19 if(fast!=NULL) 20 fast=fast->next; 21 if(slow==fast) 22 return true; 23 } 24 return false; 25 } 26 };