Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
求链表是否有环的问题，要考虑链表为空的情况，定义一个快指针和一个慢指针，如果快指针和慢指针重合就表明有环
bool hasCycle(ListNode *head){ if(head == NULL || head->next == NULL) return false; ListNode* first = head, *second = head; while(second!=NULL && second->next!=NULL){ first = first->next; second = second->next->next; if(first == second) return true; } return false; }
leetcode Linked List Cycle,布布扣,bubuko.com
原文：http://www.cnblogs.com/xiongqiangcs/p/3763897.html