[leetcode] Linked List Cycle II

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
ListNode *fast=head;
ListNode *slow=head;

while(fast&&fast->next){
fast=fast->next->next;
slow=slow->next;
if(fast==slow){//找到相遇点
break;
}
}
if(fast==nullptr||fast->next==nullptr){//没有环
return nullptr;
}
slow=head;
while(slow!=fast){
slow=slow->next;
fast=fast->next;
}
return slow;
}
};