Interview

TimeLimit:1000MS  MemoryLimit:262144KB
64-bit integer IO format:%I64d
未提交 | 登录后收藏
Problem Description
Alice and Bob are going to Tenbaba for an interview. There are totally N candidates(including Alice and Bob) applying for this job. The recruitment process of Tenbaba is strict. So all of them need to have an interview with the department manager. N candidates will get a unique integer from $1$ to $N$ with equal probability. And they will go to interview with the manager according to the number they get. The candidate with number 1 will be interviewed firstly, the candidate with number 2 will be interviewed secondly and so on.

What's more, HR(the staff to arrange the interview for you) will randomly choose a nonnegative integer $K$ in range $[0..N]$ with equal probability. The candidates whose numbers are not greater than $K$ will be interviewed in order of their numbers on the first day. Remaining candidates will be interviewed in order of their numbers on the second day. Candidates don't know $K$. But each one will knows on which day he or she will be interviewed.

Sadly, Alice forgot her number. The only thing she remember is that she will be interviewed on the second day. Alice also knows on which day Bob will be interviewed. But she doesn't know the exact number Bob has. Assume that Alice would be the $Y$-th$(1\leq Y \leq N - K)$ candidate to interview with the manager on the second day. Please help Alice to calculate the expectation of $Y$ because Alice doesn't want to go out too early.
Input
The first line is the number of test cases. It's guaranteed that the test cases is not greater than $10^5$.

Each test case contains two integers $N$ and $D$ $(2 \leq N \leq 10^9, D = \ 1 \ or \ 2$). $D = 1$ means Bob will be interviewed on the first day and $D = 2$ means Bob will be interviewed on the second day.
Output
Each test case contains one line with one integer. Let¡ˉs assume the possibility be equal to the irreducible fraction $P / Q$. Print the value of $P\cdot Q^{-1}$ in the prime field of integers modulo $1000000007(10^9+7)$.
SampleInput
3
2 1
3 2
100 1
SampleOutput
1
875000008
500000029
Submit
题目统计信息详细
总AC数0
通过人数0
尝试人数0
总提交量0
AC率0.00%
AC该题后可以添加标签
贴完标签可以获得20ACB。
并且可以获得本题所有提交代码查看权限。
点击标题可以显示标签。
如果你还没认真思考过这题,请不要查看标签
如果您已经通过了该题,请务为该题贴上标签

T^T Online Judge

[BUG反馈] [FAQ] [闽ICP备17026590号-1]
当前版本:3.24 系统时间: