1、题目描述
2、问题分析
使用快慢指针方法判断链表是否有环，然后寻找环开始的节点。
3、代码
ListNode *detectCycle(ListNode *head) {
if( head == NULL || head->next == NULL ){
return NULL ;
}
ListNode* fast = head;
ListNode* slow = head;
while( fast != NULL && slow != NULL ){
if( fast->next != NULL ){
fast = fast->next->next;
}else{
return NULL ;
}
slow = slow->next;
if( fast == slow ){
break;
}
}
if( fast == NULL || slow == NULL ){
return NULL;
}
ListNode* start = head;
while( start != slow ){
start = start->next ;
slow = slow->next;
}
return start;
}