题解

#include<bits/stdc++.h>
using namespace std;

const int N = 1005;
string f[101][101];
string add(string a,string b){
    string ans;
    int na[N] = {0},nb[N] = {0};
    int la = a.size(),lb = b.size();
    for(int i = 0;i < la;i++) na[la - 1 - i] = a[i] - '0';
    for(int i = 0;i < lb;i++) nb[lb - 1 - i] = b[i] - '0';
    int lmax = la > lb ? la : lb;
    for(int i = 0;i < lmax;i++) na[i] += nb[i],na[i + 1] += na[i] / 10,na[i] %= 10;
    if(na[lmax]) lmax++;
    for(int i = lmax - 1;i >= 0;i--) ans += na[i] + '0';
    return ans;
}
int na[N];
string mul(string a,int b){
    string ans;
    int La=a.size();
    fill(na,na + N,0);
    for(int i = La - 1;i >= 0;i--) na[La - i - 1] = a[i] - '0';
    int w = 0;
    for(int i = 0;i < La;i++) na[i] = na[i] * b + w,w = na[i] / 10,na[i] = na[i] % 10;
    while(w) na[La++] = w % 10,w /= 10;
    La--;
    while(La >= 0) ans += na[La--] + '0';
    return ans;
}
int main(){
    int n,m;
    for(int i = 1;i <= 100;i++) f[i][1] = "1";
    for(int i = 2;i <= 100;i++){
        for(int j = 1;j <= i;j++){
            f[i][j] = add(f[i - 1][j - 1],mul(f[i - 1][j],j));
        }
    }         
    while(cin >> n >> m){
        if(n < m) cout << 0 << endl;
        else cout << f[n][m] << endl;
    }
    return 0;
}