题意：有高为1,2,3,...n的杆子各一根排成一行。从左边能看到l根，从右边能看到r根，求有多少种可能(1<=l,r<=n<=20)

思路;紫书332页

#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define INF 0x3f3f3f3f
#define mod 1000000007;
using namespace std;
//    freopen("in.txt","r",stdin);
//    freopen("out.txt","w",stdout);
const int maxn = 20 + 2;
int n,l,r;
ll d[maxn][maxn][maxn];
void init()
{
    memset(d,0,sizeof(d));
    d[1][1][1] = 1;
    for(int i = 2;i <= 20;i++)
    {
        for(int j = 1;j <= i;j++)
        {
            for(int k = 1;k <= i;k++)
            {
                d[i][j][k] = d[i - 1][j - 1][k] + d[i - 1][j][k - 1] + d[i - 1][j][k] * (i - 2);
            }
        }
    }
}
int main()
{
    init();
    int T; scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d%d",&n,&l,&r);
        printf("%lld\n",d[n][l][r]);
    }
    return 0;
}