蓝桥杯 波动数列

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
using namespace std;
const int INF = 2000, MOD = (int) 1e8 + 7;
int g[INF], f[INF][INF];
int n, s, a, b;
int main() {
    scanf ("%d %d %d %d", &n, &s, &a, &b);
    int tem = (2 * n - (a * n * (n - 1) / 2 % n) - s % n ) % n;
    for (int i = 2; i <= n; i++)
        g[i] = ( (a + b) * (n - i + 1) % n);
    f[1][tem] = 1;
    for (int i = 2; i <= n; i++)
        for (int j = 0; j < n; j++) {
            f[i][j] = (f[i][j] + f[i - 1][j]) % MOD;
            f[i][ (j + g[i]) % n] += f[i - 1][j];
            f[i][ (j + g[i]) % n] %= MOD;
        }
    printf ("%d\n", f[n][0]);
}