Submission #488317

#	Time	Username	Problem	Language	Result	Execution time	Memory
488317		ExplodingFreeze	Kangaroo (CEOI16_kangaroo)	C++17	100 / 100	22 ms	22988 KiB

kangaroo

#include <bits/stdc++.h>
#define int long long
const int MOD = 1e9 + 7;
using pii=std::pair<int,int>;
using namespace std;

const int maxn = 2005;

int n, cs, cf, dp[maxn][maxn];  // dp[positions visited in first i steps][as j components] = number of ways of doing so
int32_t main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> cs >> cf;
    cs--; cf--;
    dp[1][1] = 1;
    for(int i = 1; i < n; i++)
        for(int j = 1; j <= i; j++)
        {
            if(i == cs || i == cf)
            {
                // Add at start / end as expected, creating a new component if required
                dp[i + 1][j] += dp[i][j];
                dp[i + 1][j] %= MOD;
                dp[i + 1][j + 1] += dp[i][j];
                dp[i + 1][j + 1] %= MOD;
            }
            else
            {
                // Place as a seperate component
                dp[i + 1][j + 1] += (j + 1 - (i > cs) - (i > cf)) * dp[i][j];
                dp[i + 1][j + 1] %= MOD;
                // Merge two components together
                dp[i + 1][j - 1] += (j - 1) * dp[i][j];
                dp[i + 1][j - 1] %= MOD;
            }
        }
    cout << dp[n][1] << "\n";
    return 0;
}

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