[LeetCode] 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?

class Solution {public:    bool hasCycle(ListNode *head) {        if (head == nullptr) return false;        ListNode *walker, *runner;        walker = runner = head;        while (runner->next && runner->next->next) {            walker = walker->next;            runner = runner->next->next;            if (walker == runner) return true;        }        return false;    }};