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->next!=NULL && second->next->next!=NULL){ first = first->next; second = second->next->next; if(first == second) return true; } return false; }