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

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

Solution:

快慢指針追擊問題。

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution 
{
    public boolean hasCycle(ListNode head) 
    {
        if (head == null) 
            return false;
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null)
        {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast)
                return true;
        }
        return false;
    }
}