Submission #48913

#	Time	Username	Problem	Language	Result	Execution time	Memory
48913		Benq	Kangaroo (CEOI16_kangaroo)	C++14	0 / 100	30 ms	14184 KiB

kangaroo

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;

#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)

#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()

const int MOD = 1000000007;
const ll INF = 1e18;
const int MX = 2001;

template<int SZ> struct Combo {
    ll fac[SZ+1], ifac[SZ+1];
    
    Combo() {
        fac[0] = ifac[0] = 1;
    	FOR(i,1,SZ+1) {
    	    fac[i] = i*fac[i-1] % MOD;
    	    ifac[i] = inv(fac[i]);
    	}
    }
    
    ll po (ll b, ll p) { return !p?1:po(b*b%MOD,p/2)*(p&1?b:1)%MOD; }
    ll inv (ll b) { return po(b,MOD-2); }
    
    ll comb(ll a, ll b) {
        if (a < b || b < 0 || a < 0) return 0;
        ll tmp = fac[a]*ifac[b] % MOD;
        tmp = tmp*ifac[a-b] % MOD;
        return tmp;
    }
};

Combo<MX> Co;

int dp[MX][MX];

int ad(int x, int y) { return (x+y)%MOD; }
int mul(int x, int y) { return (ll)x*y%MOD; }

void gen() {
    dp[0][0] = 1;
    FOR(i,1,MX) {
        if (i % 2 == 1) {
            FOR(j,1,i+1) dp[i][j] = ad(dp[i][j-1],dp[i-1][j-1]);
        } else {
            FORd(j,1,i) dp[i][j] = ad(dp[i][j+1],dp[i-1][j]);
        }
    }
}

int main() {
    gen();
    ios_base::sync_with_stdio(0); cin.tie(0);
    int N, cs, cf; cin >> N >> cs >> cf;
    if (cs > cf) swap(cs,cf);
    if (cs == 1) {
        cout << dp[N-1][cf-1];
        return 0;
    }
    int a = cs-2, b = cf-cs-1, c = N-cf;
    int ans = 0;
    F0R(A,a+1) {
        int mula = Co.comb(a,A);
        F0R(B,b+1) {
            int mulb = mul(mula,Co.comb(b,B));
            F0R(C,c+1) {
                ans = ad(ans,mul(mul(dp[A+1+B+C][A+1],dp[a+b+c+1-A-B-C][a+b+1-A]),mul(mulb,Co.comb(c,C))));
            }
        }
        // A+1 on left, B+C on right 
        // a+b+1-A on left, c-C on right 
    }
    cout << ans;
}

// read the question correctly (is y a vowel? what are the exact constraints?)
// look out for SPECIAL CASES (n=1?) and overflow (ll vs int?)

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