[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;
    }
}