1.由定义式直接算：n!/m!*(n-m)!

#include <iostream>
using namespace std;
long long combine(long long m,long long n ){long long result=1;for(int i=1;i<n+1;i++){//n!result*=i;}for(int i=1;i<m+1;i++){//n!/m!result/=i;}for(int i=1;i<n-m+1;i++){//n!/(m!*(n-m)!)result/=i;}return result;
}
int main(){long long n,m;cin>>m>>n;cout<<combine(m,n)<<endl;return 0;
}

2.变形式计算：

#include <iostream>
using namespace std;
long long combine(long long m,long long n ){long long result=1;for(int i=1;i<m+1;i++)result*=(n-m+i)/i;return result;
}
int main(){long long n,m;cin>>m>>n;cout<<combine(m,n)<<endl;return 0;
}