LeetCode刷题笔录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?

标准CTCI原题，快慢两个指针。

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null || head.next == null)
            return false;
        ListNode slow = head;
        ListNode fast = head;
        while(true){
            slow = slow.next;
            fast = fast.next.next;
            if(fast == null || fast.next == null)
                return false;
            if(fast == slow)
                return true;
        }
    }
}