[LeetCode]: 141: Linked List Cycle

题目：

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

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

思路：

正常的遍历，使用HashSet来判断是否重复

代码：

    public boolean hasCycle(ListNode head) {
        HashSet hashFlag = new HashSet();
        while(head!= null){
            if(!hashFlag.add(head)){
                return true;
            }
            head = head.next;
        }
        return false;
    }

思路二：采用快指针和慢指针

两个指针，移动的速度差一，所以快指针必定会“追上”慢指针。有点类似小学时候的追击问题

代码：

    public boolean hasCycle(ListNode head) {
        ListNode Fast = head;
        ListNode Slow = head;
        while(Slow!= null){
            if(Slow.next!= null && Fast.next!= null && Fast.next.next!= null){
                Slow = Slow.next;
                Fast = Fast.next.next;
                if(Slow == Fast){
                    return true;
                }
            }
            else{
                return false;
            }

        }
        return false;
    }

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