Submission #2531992

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Source Code Expand

#include <iostream>
#include <vector>

using namespace std;

const int MOD = 1e9+7;
const int MAX = 3e5+1;

int64_t fact[MAX], invfact[MAX];

int modPow(int x,int N){
    if(N == 0) return 1;
    if(N == 1) return x % MOD;
    int64_t tmp = modPow(x, N/2);
    tmp = tmp * tmp % MOD;
    if(N&1) return tmp * x % MOD;
    return tmp;
}

void init(int N){
    fact[0] = 1;
    for(int i=1;i<=N;i++){
        fact[i] = (fact[i-1]*i)%MOD;
    }
    invfact[N] = modPow(fact[N],MOD-2);
    for(int i=N-1;i>=0;i--){
        invfact[i] = invfact[i+1] * (i+1) % MOD;
    }
}

int comb(int n, int r){
    if(n-r < 0){
        return 0;
    }
    int64_t ret = fact[n] * invfact[r] % MOD;
    return (ret * invfact[n-r])%MOD;
}

int Hcomb(int n, int r){
    return comb(n+r-1,r);
}

int main(){
    int N,K; cin >> N >> K;
    init(N+K);
    cout << Hcomb(N,K) << endl;
    return 0;
}

Submission Info

Judge Result