Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1: Input: head = [3,2,0,-4], pos = 1 Output: true Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2: Input: head = [1,2], pos = 0 Output: true Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3: Input: head = [1], pos = -1 Output: false Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it using O(1) (i.e. constant) memory?
来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/linked-list-cycle 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
思路： 就是只给一个头结点然后判断它是不是环形链表，至于input里的pos好像是评测姬用的= =
就是用快慢指针，是环形的话，快指针一定会和慢指针重合，不是环形自然最后指向nullptr了……具体看链表常见问题那篇_(:з」∠)_
/** * 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* fast = head; ListNode* slow = head; bool flag = 0; while(fast != nullptr && fast->next != nullptr){ fast = fast-> next-> next; slow = slow-> next; if(fast == slow){ flag = 1; break; } } return flag; } };