Submission #1324269

#	Time	Username	Problem	Language	Result	Execution time	Memory
1324269		yousuf11	Kangaroo (CEOI16_kangaroo)	C++20	0 / 100	7 ms	15928 KiB

kangaroo

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int mod = 1e9 + 7;
int add(ll a, ll b) {
    return (a%mod + b%mod + 2*mod)%mod;
}
int mul(ll a, ll b) {
    return ((a%mod) * (b%mod)) % mod;
}
// 1 2 3
// 1 3 2
// 2 3 1
// 2 1 3
// 3 1 2
// 3 2 1
const int N = 2002;
int dp[N][N], n, st, en;
int rec(int i, int j) {
    if (j < 0) return 0;
    if (i == n) return (j == 1);
    int&ret = dp[i][j];
    // add new
    if (i == st || i == en) {
        ret = rec(i + 1, j + 1);
        // push_front , back
        if (j)
            ret = add(ret, rec(i + 1, j));
    }
    else {
        ret = mul(rec(i + 1, j + 1), j + 1 - (i > st) - (i > en));
        // merge
        if (j >= 2)
            ret = add(ret, mul(rec(i + 1, j - 1), j - 1));
    }

    return ret;
}
// [] -> inc {i} -> dec [] -> always merge works -> there is no insert
// push_back -> cant do 2 i,i+1
inline void die() {
    memset(dp, -1, sizeof(dp));
    cin >> n >> st >> en;
    --st, --en;
    cout << rec(0,0) << "\n";
}

signed main() {
    ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int t = 1;
    // cin >> t;
    while (t--)
        die();
}

Compilation message (stderr)

kangaroo.cpp: In function 'int main()':
kangaroo.cpp:51:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   51 |     freopen("input.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
kangaroo.cpp:52:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   52 |     freopen("output.txt", "w", stdout);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 0 / 6
• Subtask 2: 0 / 30
• Subtask 3: 0 / 15
• Subtask 4: 0 / 49