[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;
do {
if (fast == NULL || fast->next == NULL) return NULL;
fast = fast->next->next;
slow = slow->next;
} while (fast != slow);
ListNode *n1 = head;
ListNode *n2 = slow;
while (n1 != n2) {
n1 = n1->next;
n2 = n2->next;
}
return n1;
}
};