[LeetCode] Linked List Cycle

[LeetCode] Linked List Cycle

 

Linked List Cycle

 

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

Solution:

1. The most basic method is to use a set to store all existing pointers. If a duplicate occurs, a ring exists. If no duplicate exists, no ring exists.

 

/** * 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* p = head;        set
 
   s;        while(p!=NULL){            if(s.find(p)!=s.end()){                return true;            }            s.insert(p);            p=p->next;        }        return false;    }};
 

2. Double pointer method. Set two pointers. one pointer takes one step at a time and the other pointer takes two steps at a time. If two pointers meet each other, a ring exists. If you do not meet each other, it indicates that there is no ring. See specific: http://blog.csdn.net/kangrydotnet/article/details/45154927

 

 

/** * 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* one = head;        ListNode* two = head;        while(two!=NULL && two->next!=NULL){            one=one->next;            two=two->next->next;            if(one==two){                return true;            }        }        return false;    }};