逆元求组合数----模板

#include <stdio.h>
#include <iostream>
using namespace std;
const int MOD=998244353;
const int MAXN=202020;
int fac[MAXN],facinv[MAXN];
 
long long quickmul(int a,int b)
{
	long long ret=1;
	for(; b ; b >>=1 ,a =(long long) a * a % MOD)
		if((b & 1))
			ret=ret * a % MOD;
	return ret;
}
long long C(int n,int m)
{
	if(n<0||m<0||n<m)
		return 0;
	return (long long)fac[n]*facinv[m]%MOD*facinv[n-m]%MOD;
} 
void init()
{
	fac[0]=1;
	for(int i=1;i<=MAXN;i++)
		fac[i]=(long long)fac[i-1]*i%MOD;
	facinv[MAXN]=quickmul(fac[MAXN],MOD-2);
	for(int i=MAXN;i>0;i--)
		facinv[i-1]=(long long)facinv[i]*(i)%MOD;
		//1/(i-1)!=i/(i)!
}
int main()
{
	init();
	int n,m;
	while(1)
	{
		scanf("%d %d",&n,&m);
		printf("%lld\n",C(n,m));
	}
}