141. Linked List Cycle

1. 问题描述

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
Tags: Linked List Two Pointers
Similar Problems: (M) Linked List Cycle II

2. 解题思路

• 画图分析，发现规律，两个指针（*pF 步长为1，*pS步长为2）

3. 代码

class Solution {
public:
    bool hasCycle(ListNode *head) 
    {
        if (!head)
        {
            return false;
        }
        ListNode *pF = head;
        ListNode *pS = head;
        while (pS->next)
        {
            pS = pS->next;
            if (pS == pF)
            {
                return true;
            }
            pS = pS->next;
            if (!pS)
            {
                return false;
            }
            pF = pF->next;
        }
        return false;
    }
};

4. 反思