【HDU 5698】瞬间移动（组合数,逆元）

#include<cstdio>
#define M 1000000007
#define N 200001
#define ll long long
ll fac[N]={1,1},inv[N]={1,1},f[N]={1,1};
int n,m;
ll C(ll a,ll b){
    return fac[a]*inv[b]%M*inv[a-b]%M;
}
int main(){
    for(int i=2;i<N;i++){
        fac[i]=fac[i-1]*i%M;
        f[i]=(M-M/i)*f[M%i]%M;
        inv[i]=inv[i-1]*f[i]%M;
    }
    while(~scanf("%d%d",&n,&m))
        printf("%lld\n",C(m+n-4,m-2));
}