HDU 6333:Harvest of Apples

Problem B. Harvest of Apples

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 262144/262144 K (Java/Others)
Total Submission(s): 347    Accepted Submission(s): 121

 

Problem Description

There are $n$ apples on a tree, numbered from $1$ to $n$.
Count the number of ways to pick at most $m$ apples.

 

 

Input

The first line of the input contains an integer $T$ $(1 \le T \le 10 ^ 5)$ denoting the number of test cases.
Each test case consists of one line with two integers $n, m$ $(1 \le m \le n \le 10 ^ 5)$.

 

 

Output

For each test case, print an integer representing the number of ways modulo $10 ^ 9 + 7$.

 

 

Sample Input

2 5 2 1000 500

 

 

Sample Output

16 924129523

 

 

Source

2018 Multi-University Training Contest 4

 

 

Recommend

chendu

 

分析：这题显然暴力O(n^2)是要TLE的，比赛的时候考虑简单数学优化一下变成2.5e9，显然肯定也是要TLE的。后来考虑深度数学优化得出：

其中F1表示超几何函数。。。然而找了半个多小时没有找到C++的模板。。就很绝望

补题的时候发现可以用莫队和分块去搞，学了半天嘤嘤嘤，太菜了。

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