Submission #1093144

#	Time	Username	Problem	Language	Result	Execution time	Memory
1093144		Zero_OP	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	15 ms	14212 KiB

kangaroo

#include "bits/stdc++.h"

using namespace std;

const int mod = 1e9 + 7;

void add(int& a, int b){
    a += b;
    if(a >= mod) a -= mod;
}

void sub(int& a, int b){
    a -= b;
    if(a < 0) a += mod;
}

int mul(int a, int b){
    return 1LL * a * b % mod;
}

const int MAX = 2e3 + 5;

int n, s, t, dp[MAX][MAX];

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    cin >> n >> s >> t;

    dp[0][0] = 1;
    for(int i = 0; i < n; ++i){
        for(int j = 0; j <= i; ++j){
            if(!dp[i][j]) continue;

            if(i + 1 == s || i + 1 == t){
                add(dp[i + 1][j], dp[i][j]);
                add(dp[i + 1][j + 1], dp[i][j]);
            } else{
                add(dp[i + 1][j + 1], mul(dp[i][j], j - 1 + (i + 1 < s) + (i + 1 < t)));
                if(j > 1) add(dp[i + 1][j - 1], mul(dp[i][j], j - 1));
            }
        }
    }

    cout << dp[n][1] << '\n';

    return 0;
}

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