141. Linked List Cycle 判断链表中是否存在“环”

141. Linked List Cycle

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Given a singly linked list, determine if it is a palindrome.

Follow up:
Could you do it in O(n) time and O(1) space?

快慢指针

1. 快指针从head+1出发，慢指针从head出发， 当快能够追上慢，说明有环
2. 需判断头结点/头指针为空的清情形

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == nullptr || head->next == nullptr)
            return 0;
        
        ListNode* pSlow = head;
        ListNode* pFast = head->next;
        
        while (pFast != nullptr && pSlow != nullptr)
        {
            if (pFast == pSlow)
                return 1;
            
            pSlow = pSlow->next;
            pFast = pFast->next;
            
            if (pFast != nullptr)
                pFast = pFast->next;
        }
        return 0;
    }
};