HDU 4828 Grids

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
#define LL long long
const int maxn = 1000010;
const LL mod = 1000000007;

LL ans[maxn];

void exgcd(LL a, LL b, LL &x, LL &y)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        return ;
    }
    exgcd(b, a%b, x, y);
    LL t = x;
    x = y;
    y = t - a/b * x;
}

void init()
{
    ans[1] = 1;
    for(int i = 2; i < maxn; i++)
    {
        ans[i] = (ans[i-1] * (4 * i - 2)) % mod;
        LL x, y;
        exgcd((LL)i+1, mod, x, y);
        ans[i] = (ans[i] * ((x + mod) % mod)) % mod;
    }
    return ;
}


int main()
{
    int t;
    cin >> t;
    init();
    for(int Case = 1; Case <= t; Case++)
    {
        int n;
        scanf("%d", &n);
        printf("Case #%d:\n%I64d\n", Case, ans[n]);
    }
}