Submission #972765

#	Time	Username	Problem	Language	Result	Execution time	Memory
972765		Beerus13	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	12 ms	14192 KiB

kangaroo

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int N = 2e3 + 5;
const int mod = 1e9 + 7;

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

int n, sta, fin;
int dp[N][N];

void solve() {
    cin >> n >> sta >> fin;
    dp[0][0] = 1;
    for(int i = 1; i <= n; ++i) {
        for(int j = 1; j <= i; ++j) {
            if(i == sta || i == fin) {
                add(dp[i][j], (dp[i - 1][j - 1] + dp[i - 1][j]) % mod);
            }
            else {
                int res = j - (i > sta) - (i > fin);
                add(dp[i][j], (1ll * dp[i - 1][j - 1] * res + 1ll * j * dp[i - 1][j + 1]) % mod);
            }
        }
    }
    cout << dp[n][1] << '\n';
}

signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    int test = 1;
    // cin >> test;
    while(test--) solve();
    return 0;
}

// https://oj.uz/problem/view/CEOI16_kangaroo

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