bzoj 4402 Claris的剑 组合数学

#include<cstdio>#include<iostream>#define LL long long #define maxn 4000005using namespace std;int N=4000000;LL mod=1e9+7;LL fac[maxn],inv[maxn];LL pow(LL x,int y){    if(y==0) return 1;    if(y==1) return x;    LL a=pow(x,y/2);    a=(a*a)%mod;    if(y%2) return (a*x)%mod;    return a;}void init(){    fac[0]=1;    for(int i=1;i<=N;i++)        fac[i]=(fac[i-1]*i)%mod;    inv[N]=pow(fac[N],mod-2)%mod;    for(int i=N-1;i>=1;i--)        inv[i]=(inv[i+1]*(i+1))%mod;    inv[0]=1;}LL calc(int n,int m){    if(n<0) return 0;    n>>=1;m--;    LL ans=fac[m+n];    ans*=inv[m];ans%=mod;    ans*=inv[n];ans%=mod;    return ans;}int main(){    init();    int n,m;    scanf("%d%d",&n,&m);    LL ans=0;    if(n&&m) ans++;    m=min(n,m);    for(int i=2;i<=m;i++)    {        ans+=calc(n-i,i);        ans+=calc(n-i-1,i);ans%=mod;    }    cout<<(ans%mod)<<endl;}