LeetCode 141. Linked List Cycle

题目链接

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?

解：

满足 without using extra space这个要求的话，可以只用2个指针来判断一个链表中的环，快的指针一次走2个结点，慢指针一次走一个结点，若有环路，两指针会先后进入环路，在慢指针被快指针“扣圈”时两指针在环路上相遇，代码：

/** * 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) {        ListNode* faster, *slower;        if (head == NULL) return false;        faster = slower = head;        while (true) {            slower = slower->next;            faster = faster->next;            if (faster == NULL) return false;            faster = faster->next;            if (faster == NULL) return false;            if (slower == faster) return true;        }    }};