Submission #1325883

#	Time	Username	Problem	Language	Result	Execution time	Memory
1325883		kareemessam	Kangaroo (CEOI16_kangaroo)	C++20	100 / 100	50 ms	16300 KiB

kangaroo

#include <bits/stdc++.h>
// #include "debug.h"
using namespace std;

using ll = long long;
using ld = long double;

const int N = 2e3 + 5 , M = 1e9 + 7;


int add(ll a , int b) {
    return (a + b + M) % M;
}

int mul(ll a , int b) {
    return a * b % M;
}

int dp[N][N] , st , en , n;

int solve(int i , int j) {
    if (j < 0) return 0;
    if (i == n)
        return j == 1;

    auto &ret = dp[i][j];
    if (~ret)
        return ret;

    ret = 0;


    // forced one position
    if (i == st || i == en) {

        ret = add(ret , solve(i + 1 , j));
        ret = add(ret , solve(i + 1 , j + 1));
    } else {
        // start a new comp
        ret = add(ret , mul(j + 1 - (i > st) - (i > en) , solve(i + 1 , j + 1)));

        if (j >= 2) // merge two comps
            ret = add(ret , mul(j - 1 , solve(i + 1 , j - 1)));
    }

    return ret;

}

void solve() {
    cin >> n >> st >> en;
    st-- , en--;
    cout << solve(0 , 0);
}

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(nullptr);
    memset(dp, -1, sizeof dp);

    int tc = 1;
    // cin >> tc;
    while (tc--) solve();
}

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