[leetcode]141. 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?

转自https://leetcode.com/discuss/32906/o-1-space-solution

方法：

使用两个指针：slow和fast。它俩都从head出发，fast指针每次走两步，slow指针每次走一步。若没有循环，一定fast指针率先遇到null；若有循环，fast和slow一定能相遇。

/**
 * 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){
            if(fast.next == slow){
                return true;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        return false;
    }
}