Submission #393923

#	Time	Username	Problem	Language	Result	Execution time	Memory
393923		KhaledFarhat	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	29 ms	16072 KiB

kangaroo

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

const int MOD = 1000000007;

int Add(int x, int y) {
    return (x + y) % MOD;
}

void Increase(int& x, int y) {
    x = Add(x, y);
}

int Multiply(int x, int y) {
    return 1LL * x * y % MOD;
}

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

    int n, startPosition, finishPosition;
    cin >> n >> startPosition >> finishPosition;

    vector<vector<int>> ways(n + 2, vector<int>(n + 1));

    ways[2][1] = 1;

    for (int i = 2; i <= n; ++i) {
        for (int components = 1; components < i; ++components) {
            if (ways[i][components] == 0) {
                continue;
            }

            if (i == startPosition || i == finishPosition) {
                Increase(ways[i + 1][components + 1], ways[i][components]); // open a new component on the side
                Increase(ways[i + 1][components], ways[i][components]); // append to the side
            }
            else {
                // create a new component
                int validPosition = (components + 1) - (i > startPosition) - (i > finishPosition);
                Increase(ways[i + 1][components + 1], Multiply(validPosition, ways[i][components]));

                // merge two component
                validPosition = components - 1;
                Increase(ways[i + 1][components - 1], Multiply(validPosition, ways[i][components]));
            }
        }
    }

    cout << ways[n + 1][1];

    return 0;
}

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