盒子与小球之二

#include<cstdio>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<map>
#include<cstring>
#define DEBUG(x) cout << #x << " = " << x << endl
typedef long long ll;
using namespace std;
int N,A,B;
ll ways[25][20][20];
///将a个红球和b个蓝球放入前i个盒子中的放法
ll Ways(int i,int a,int b)
{
    ll t=ways[i][a][b];
    if(t!=-1)return t;
    if(i==0)return 1;
    if(a==0&&b==0)return 1;
    ll r;
    ///不放
    r=Ways(i-1,a,b);
    ///放一种
    for(int k=1;k<=a;k++){
        r+=Ways(i-1,a-k,b);
    }
    for(int k=1;k<=b;k++){
        r+=Ways(i-1,a,b-k);
    }
    ///放两种
    for(int p=1;p<=a;p++){
        for(int q=1;q<=b;q++){
            r+=Ways(i-1,a-p,b-q);
        }
    }
//    DEBUG(r);
    ways[i][a][b]=r;
    return r;
}
int main()
{
//    freopen("in.txt","r",stdin);
    memset(ways,0xff,sizeof(ways));
    scanf("%d %d %d",&N,&A,&B);
    printf("%lld\n",Ways(N,A,B));
    return 0;
}