Submission #873898

#	Time	Username	Problem	Language	Result	Execution time	Memory
873898		noiaint	Kangaroo (CEOI16_kangaroo)	C++17	100 / 100	30 ms	14172 KiB

kangaroo

#include <bits/stdc++.h>
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define getbit(x, i) (((x) >> (i)) & 1)
#define bit(x) (1 << (x))
#define file ""
using namespace std;

mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
long long rand(long long l, long long r) {
    return l + rd() % (r - l + 1);
}

const int N = 2e3 + 5;
const int mod = 1e9 + 7; // 998244353;
const int inf = INT_MAX;
const long long llinf = LLONG_MAX;
const int lg = 25; // lg + 1

template<class X, class Y> bool minimize(X &a, Y b) {
    return a > b ? (a = b, true) : false;
}
template<class X, class Y> bool maximize(X &a, Y b) {
    return a < b ? (a = b, true) : false;
}
template<class X, class Y> bool add(X &a, Y b) {
    a += b;
    while (a >= mod) a -= mod;
    while (a < 0) a += mod;
    return true;
}

int n, s, e;
int f[N][N];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    // freopen(file".inp", "r",stdin);
    // freopen(file".out", "w",stdout);

    cin >> n >> s >> e;

    f[1][1] = 1;

    for (int i = 2; i <= n; ++i) {
    	for (int j = 1; j <= i; ++j) {
    		if (i == s || i == e) {

    			// one place we can put i

    			// create a new one
    			add(f[i][j], f[i - 1][j - 1]);
    			
    			// add to a connect component
    			add(f[i][j], f[i - 1][j]);
    		}
    		else {
    			
    			// combine 2 components into 1, i is the largest at this moment, so there're j places to put i in j + 1 components
    			add(f[i][j], 1LL * f[i - 1][j + 1] * j % mod);
    			
    			// create a new component with one element
    			// if s was placed down, i must be greater than s, so do e
    			// so, we must subtract 2 places s and e if placed down
    			add(f[i][j], 1LL * f[i - 1][j - 1] * (j - (i > s) - (i > e)) % mod);
    		}
    	}
    }

    cout << f[n][1];
    
    return 0;
}

/*
restriction
a[i] < a[j] > a[k]
*/

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