solve.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
struct ListNode {
	int val;
	struct ListNode *next;
};
bool hasCycle(struct ListNode *head) {
	if (head == NULL)
	    return false;
	struct ListNode *fast = head, *slow = head;
	while (fast && fast->next) {
		fast = fast->next->next;
		slow = slow->next;
		if (fast == slow)
			return true;
	}
	return false;
}
struct ListNode *detectCycle(struct ListNode *head)
{
	if (head == NULL)
		return NULL;
	struct ListNode *fast = head, *slow = head;
	while (fast && fast->next) {
		fast = fast->next->next;
		slow = slow->next;
		if (fast == slow) {
			fast = head;
			while (fast != slow) {
				fast = fast->next;
				slow = slow->next;
			}
			return fast;
		}
	}
	return NULL;
}
void mk_list(struct ListNode **head, int a[], int n)
{
	struct ListNode *p = malloc(sizeof(*p));
	*head = p;
	p->val = a[0];
	p->next = NULL;
	for (int i = 1; i < n; ++i)
	{
		struct ListNode *t = malloc(sizeof(*t));
		t->val = a[i];
		t->next = NULL;
		p->next = t;
		p = t;
	}
}
int main(int argc, char **argv)
{
	struct ListNode *head;
	int a[] = {1,2,3,4,5};
	mk_list(&head, a, 5);
	//head->next->next = head;
	printf("%d\n", hasCycle(head));
	printf("%d\n", detectCycle(head)->val);
	return 0;
}