Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
Follow up:
Can you solve it without using extra space?
Hash表解法，代码：
class Solution { public: ListNode *detectCycle(ListNode *head) { if(head==NULL) return NULL; if(head->next==NULL) return NULL; if(head->next==head) return head; map<ListNode*,bool> m; ListNode *p=head; ListNode *res=NULL; while (p!=NULL) { if(m.count(p)==0){m[p]=true;p=p->next;} else {res=p;break;} } return res; } };
原文：http://www.cnblogs.com/fightformylife/p/4245916.html