Submission #2528756

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Source Code Expand

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstring>
#include <assert.h>
#include <sys/time.h>
#include <fstream>
#include <iomanip>

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  FOR(i,0,n)
#define REP(i,n)  FOR(i,0,n)
#define each(i,c) for(auto i=(c).begin(); i!=(c).end(); ++i)
#define EACH(i,c) for(auto i=(c).begin(); i!=(c).end(); ++i)
#define exist(s,e) ((s).find(e)!=(s).end())

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
#define deb(x) cerr << #x << " = " << (x) << " , ";
#define debl cerr << " (L" << __LINE__ << ")"<< endl;
#define sz(s) (int)((s).size())


#define clr(a) memset((a),0,sizeof(a))
#define nclr(a) memset((a),-1,sizeof(a))
#define pb push_back
#define INRANGE(x,s,e) ((s)<=(x) && (x)<(e))
#define MP(x,y) make_pair((x),(y))

double pi=3.14159265358979323846;

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<string> vs;

template<typename T> std::ostream& operator<<(std::ostream& os, const vector<T>& z){
	os << "[ ";
	REP(i,z.size())os << z[i] << ", " ;
	return ( os << "]" << endl);
}

template<typename T> std::ostream& operator<<(std::ostream& os, const set<T>& z){
	os << "set( ";
	EACH(p,z)os << (*p) << ", " ;
	return ( os << ")" << endl);
}

template<typename T,typename U> std::ostream& operator<<(std::ostream& os, const map<T,U>& z){
	os << "{ ";
	EACH(p,z)os << (p->first) << ": " << (p->second) << ", " ;
	return ( os << "}" << endl);
}

template<typename T,typename U> std::ostream& operator<<(std::ostream& os, const pair<T,U>& z){
	return ( os << "(" << z.first << ", " << z.second << ",)" );
}

double get_time(){
	struct timeval tv;
	gettimeofday(&tv, NULL);
	return tv.tv_sec + tv.tv_usec*1e-6;
}

typedef unsigned int uint32_t;
struct RND{
	uint32_t x;
	uint32_t y;
	uint32_t z;
	uint32_t w;
	RND(){
		x=123456789;
		y=362436069;
		z=521288629;
		w=88675123;
	}
	void init(int seed){
		x=123456789;
		y=362436069;
		z=521288629;
		w=seed+100;
		REP(i,10)get();
	}
	uint32_t get(){
		uint32_t t;
		t=x^(x<<11);
		x=y;y=z;z=w;
		w=(w^(w>>19))^(t^(t>>8));
		return w;
	}
};
RND rnd;

ll double_to_ll(double d){
	if(d>0)return d+0.5;
	return d-0.5;
}

ll mod=1000000007;
struct mint{
	ll value;
	mint():value(0){}
	mint(ll v):value((v%mod+mod)%mod){}
};
mint& operator+=(mint&a, mint b){return a=a.value+b.value;}
mint& operator-=(mint&a, mint b){return a=a.value-b.value;}
mint& operator*=(mint&a, mint b){return a=a.value*b.value;}
mint operator+(mint a, mint b){return a+=b;}
mint operator-(mint a, mint b){return a-=b;}
mint operator*(mint a, mint b){return a*=b;}
mint operator-(mint a){return 0-a;}
bool operator==(mint a, mint b){return a.value==b.value;}
bool operator!=(mint a, mint b){return a.value!=b.value;}


std::ostream& operator<<(std::ostream& os, const mint& m){
return ( os << m.value );}
ll extgcd(ll a, ll b, ll &x, ll &y){
	ll d=a;
	if(b!=0){
		d=extgcd(b, a%b, y, x);
		y-=(a/b)*x;
	}
	else{
		x=1,y=0;
	}
	return d;
}
ll modinverse(ll a, ll b){
	ll x,y;
	ll d=extgcd(a,b, x, y);
	assert(d==1);
	return (x%b+b)%b;
}
mint& operator/=(mint&a, mint b){return a=a.value*modinverse(b.value,mod);}
mint operator/(mint a, mint b){return a/=b;}

const int D=2000010;
mint factorial[D];
mint invfactorial[D];
mint pow2[D];

struct Precomp{
	Precomp(){
		factorial[0]=1;
		for(ll k=1; k<D; k++)factorial[k].value = factorial[k-1].value*k%mod;
		invfactorial[D-1] = mint(1) / factorial[D-1];
		for(ll k=D-2; k>=0; k--) invfactorial[k].value = invfactorial[k+1].value*(k+1)%mod;
		pow2[0]=1;
		for(ll k=1; k<D; k++)pow2[k].value = pow2[k-1].value*2%mod;

	}
}precomp;

mint comb(ll n, ll a){
	if(0<=a && a<=n)return factorial[n] * invfactorial[a] * invfactorial[n-a];
	else return mint(0);
}

mint Rcomb(ll n, ll r){
	// x_1+...+x_r= nの個数
	if(n==0 && r>=0)return 1;
	return comb(n+r-1, r-1);
}


ll memo[2010][2010];

mint rec(int n, int i){
	assert(0<=i && i<n);
	 ll &ret = memo[n][i];
	if(ret != -1) return mint(ret);
	mint ans;
	if(n==1 || n==2){
		ans = 1;
	}
	else if(i==0){
		ans = 2*rec(n-1, 0);
	}
	else if(i==n-1 || i==n-2){
		ans = comb(2*(n-1),n-1) / n ;
	}
	else{
		ans = rec(n-1, i)*2 + rec(n,i-1)/2;
	}
	return ret = ans.value;
}

void _main(istream &inp){
	nclr(memo);
	ll n,k;
	inp >> n >> k;
	cout << rec(n,k-1) << endl;
//	FOR(n,17,18){
//		rep(i,n){
//			deb(n);deb(i);deb(rec(n,i));debl;
//		}
//	}
//	cout << fixed << setprecision(12);
//	while(true){
//		break;
//	}

}

int main(){
	if(0){
		ifstream ifs("test.txt");
		_main(ifs);
	}
	else{
		_main(cin);
	}
	return 0;
}

Submission Info

Judge Result