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簡單求組合數(除法取模)

#include<bits/stdc++.h>
using namespace std;
#define inf 0x3f3f3f3f
const int maxn=1e5+9;
#define LL long long
int e_gcd(int a,int b,int & x,int &y){
    if(!b){
        x=1;
        y=0;
        return a;
    }
    int gcd=e_gcd(b,a%b,y,x);
    y-=x*a/b;
    return gcd;
}
int inv(int a,int mod){
    int x,y;
    int d=e_gcd(a,mod,x,y);
    if(d!=1)return -1;
    int ans=(x%mod+mod)%mod;
    return ans;
}
LL C(LL n,LL m,LL mod){
    if(m<0)return 0;
    else if(m>n)return 0;
    if(m>n-m)m=n-m;
    LL up=1,down=1;
    for(int i=0;i<m;i++){
        up=up*(n-i)%mod;
        down=down*(i+1)%mod;
    }
    if(inv(down,mod)==-1){
        return up%(down*mod)/down;
    }
    else
    return up*inv(down,mod)%mod;
}
int main(){
    int n,m,mod;
    cin>>n>>m>>mod;
    //cout<<inv(n,m)<<endl;
    cout<<C(n,m,mod)<<endl;
}