列舉當前這些數字中比第一個數字大的有多少

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll dp[35][1005];
ll C[35][35];
const ll mod = 1e9 + 7;

ll dfs(int n, int m)
{
    //cout << n << ' ' << m << endl;
    if (n <= 0 || m <= 0)
        return 0;
    if (dp[n][m] != -1)
    {
        return dp[n][m];
    }
    if (m < n)
        return dp[n][m] = 0;
    if (m > n * (n + 1) / 2)
        return dp[n][m] = 0;
    ll &ans = dp[n][m];
    ans = 0;
    ans = (dfs(n - 1, m - n) * 2) % mod;
    for (int i = 1; i <= n - 2; i++)
    {
        for (int j = 1; j <= m - n - 1; j++)
        {
            ll ans1 = dfs(i, j);
            ll ans2 = dfs(n - i - 1, m - j - n);
            if (!ans1 || !ans2)
                continue;
            ans = (ans + C[n - 1][i] * ans1 % mod * ans2 % mod) % mod;
        }
    }
    return ans;
}

int main()
{
    memset(dp, -1, sizeof(dp));
    C[0][0] = 1;
    for (int i = 1; i <= 30; i++)
    {
        C[i][0] = 1;
        for (int j = 1; j <= 30; j++)
            C[i][j] = (C[i - 1][j] + C[i - 1][j - 1]) % mod;
    }
    int n, m;
    dp[1][1] = 1;
    while (scanf("%d%d", &n, &m) == 2)
    {
        printf("%lld\n", dfs(n, m));
    }
    return 0;
}