思路 10w询问，a、b范围2k 递推

• 证明
  

#include <iostream>

using namespace std;

const int N = 2010, mod = 1e9 + 7;
int n, a, b;
int c[N][N];

void init()
{
    for (int i = 0; i < N; i ++ )
        for (int j = 0; j <= i; j ++ )
            if (!j) c[i][j] = 1; //在可选的苹果中选0个，就是不选，也是一种选择
            else c[i][j] = (c[i - 1][j - 1] + c[i - 1][j]) % mod;
}

int main()
{
    cin >> n;

    init();

    while (n -- )
    {
        int a, b;
        scanf("%d%d", &a, &b);
        printf("%d\n", c[a][b]);
    }

    return 0;
}