Submission #470372

#	Time	Username	Problem	Language	Result	Execution time	Memory
470372		hoanghq2004	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	22 ms	14028 KiB

kangaroo

#include <bits/stdc++.h>

using namespace std;

const int mod = 1e9 + 7;
const int Nmax = 2010;

int n;
int S, T;
int f[Nmax][Nmax];

int main() {
    cin >> n >> S >> T;
    f[1][1] = 1;
    for (int i = 1; i < n; ++i) {
        for (int j = 1; j <= i; ++j) {
            if (f[i][j] == 0) continue;
            if (i + 1 == S || i + 1 == T) {
                f[i + 1][j] += f[i][j];
                f[i + 1][j + 1] += f[i][j];
                if (f[i + 1][j] >= mod) f[i + 1][j] -= mod;
                if (f[i + 1][j + 1] >= mod) f[i + 1][j + 1] -= mod;
            } else {
                f[i + 1][j - 1] += 1LL * f[i][j] * (j - 1) % mod;
                if (f[i + 1][j - 1] >= mod) f[i + 1][j - 1] -= mod;
                f[i + 1][j + 1] += 1LL * (j + 1 - (i + 1 > S) - (i + 1 > T)) * f[i][j] % mod;
                if (f[i + 1][j + 1] >= mod) f[i + 1][j + 1] -= mod;
            }
        }
    }
    cout << f[n][1];
}

• Subtask 1: 6 / 6
• Subtask 2: 30 / 30
• Subtask 3: 15 / 15
• Subtask 4: 49 / 49