代码

#include <bits/stdc++.h>
using namespace std;
const int N = 2010, mod = 1e9 + 7;
int c[N][N];
int main()
{
    int n; 
    scanf("%d", &n);
    for(int i = 0; i < N; i++) 
        for(int j = 0; j <= i; j++)    
            if(!j) c[i][j] = 1; 
            else c[i][j] = (c[i - 1][j - 1] + c[i - 1][j]) % mod;
    while(n--)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        printf("%d\n", c[a][b]);
    }
}

核心

数学知识