#include
#include
#include
using namespace std;
#define REP(i,n) for(int i=0;i<(n);++i)
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)
#define RFOR(i,a,b) for(int i=(a);i>=(b);--i)
typedef long long LL;
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* n1 = head;
ListNode* n2 = head;
while (true) {
n1 = n1->next;
if (n1 == NULL) return NULL;
n2 = n2->next;
if (n2 == NULL) return NULL;
n2 = n2->next;
if (n2 == NULL) return NULL;
if (n1 == n2) {
n1 = head;
while (n1 != n2) {
n1 = n1->next;
n2 = n2->next;
}
return n1;
}
}
}
};
int main() {
return 0;
}