【面试准备】letcode-Linked List Cycle ||

/**
* 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) {
if(head == NULL){
return NULL;
}
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 slow;
}
};