用兩個指標，一個快指標一次走兩步，一個慢指標一次走一步。快慢指標可以重合表示連結串列有環，此時距離環起點的距離和起點距離環起點的距離相等。

#include "bits/stdc++.h"
using namespace std;
struct List {
    List* next;
};
List* beginOfCircle(List* p1, List* p2) {
    while (p1 != p2) {
        p1 = p1->next;
        p2 = p2->next;
    }
    return p1;
}
List* hasCircle(List* head) {
    List
 
* fast = head;
    List* slow = head;
    while (slow != NULL && fast != NULL && fast->next != NULL) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) {
            return beginOfCircle(head, slow);
        }
    }
    return NULL;
}
List
 
* init() {
    List* rec = (List*)malloc(sizeof(List));
    rec->next = NULL;
}
int main() {
    List* head;
    List* ans;
    List* now;
    List* res;
    /*
    head = (List*)malloc(sizeof(List));
    head->next = (List*)malloc(sizeof(List));
    head->next->next = (List*)malloc(sizeof(List));;
    head->next->next->next = (List*)malloc(sizeof(List));
    head->next->next->next = head->next;
    res = hasCircle(head);
    if (res == head->next) {
        puts("YES");
    } else {
        puts("NO");
    }
    
 
*/
    int n, m;
    scanf("%d %d", &n, &m);
    head = init();
    now = head;
    while (--n) {
        now->next = init();
        now = now->next;
    }
    ans = now;
    if (m == 0) {
        res = hasCircle(head);
        if (res == ans) {
            puts("YES");
        } else {
            puts("NO");
        }
        return 0;
    }
    while (--m) {
        now->next = init();
        now = now->next;
    }
    now->next = ans;
    res = hasCircle(head);
    if (res == ans) {
        puts("YES");
    } else {
        puts("NO");
    }
    return 0;
}

main函式裡為驗證，可以輸入環起點的編號n和一個環的長度m。最後返回的結果等於環起點輸出“YES”，當連結串列中無環的情況下返回NULL輸出結果為“NO”;