[Leetcode] 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?

 

[Thoughts]

有一种使用两个指针的方法，一个fast指针，每次移动两步；一个slow指针，每次移动一步。如果两个指针能相遇，则链表具有一个环形。

public boolean hasCycle(ListNode head) 

{

    ListNode faster = head;

    ListNode slower = head;



    while (faster != null)

    {

        faster = faster.next;

        if (faster == null)

        {

            return false;

        }

        else

        {

            faster = faster.next;

        }

        slower = slower.next;

        if (faster == slower)

        {

            return true;

        }

    }

    return false;

}

 

另一种方法(只是开玩笑)

public class Solution {

    public boolean hasCycle(ListNode head) {

        if(head == null)

            return false;

        int i = 0;

        while(head.next != null){

            i++;

            if (i > 10000){

                return true;

            }

            head = head.next;

        }

        return false;

    }

}