1043 幸运号码 数位DP

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <assert.h>
#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;


#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <bitset>
const int MOD = 1e9 + 7;
LL dp[2][9 * 1000 + 20];
void work() {
    int n;
    cin >> n;
    dp[0][0] = 1;
    int now = 0;
    for (int i = 1; i <= n; ++i) {
        now = !now;
        for (int k = 0; k <= 9 * i; ++k) {
            LL sum = 0;
            for (int j = 0; j <= 9; ++j) {
                if (k >= j) {
                    sum += dp[!now][k - j];
                    if (sum >= MOD) sum %= MOD;
                }
            }
            dp[now][k] = sum;
        }
    }
    LL ans = 0;
    for (int i = 1; i <= 9 * n; ++i) {
//        assert(dp[now][i] - dp[!now][i] >= 0);
        ans += ((dp[now][i] - dp[!now][i] + MOD) % MOD * dp[now][i]) % MOD;
        if (ans >= MOD) ans %= MOD;
    }
    cout << ans << endl;
}

int main() {
#ifdef local
    freopen("data.txt", "r", stdin);
//    freopen("data.txt", "w", stdout);
#endif
    work();
    return 0;
}