leetcode_c++：链表：Linked List Cycle II(142)

class Solution {
public:
ListNode *detectCycle(ListNode *head) {
ListNode *slow = head;
ListNode *fast = head;
while(fast != NULL && fast->next != NULL){
slow = slow->next;
fast = fast->next->next;
if(slow == fast)
break;
}
if(fast == NULL || fast->next == NULL)
return NULL;
slow = head;
while(slow != fast){
slow = slow->next;
fast = fast->next;
}
return fast;
}

};