Codeforces Round 647 (Div. 1) - Thanks, Algo Muse!
E
James and the Chase
Codeforces
3000
原题
← DF →
翻译中…

James Bond has a new plan for catching his enemy. There are some cities and directed roads between them, such that it is possible to travel between any two cities using these roads. When the enemy appears in some city, Bond knows her next destination but has no idea which path she will choose to move there.

翻译中…

The city $a$ is called interesting, if for each city $b$, there is exactly one simple path from $a$ to $b$. By a simple path, we understand a sequence of distinct cities, such that for every two neighboring cities, there exists a directed road from the first to the second city.

翻译中…

Bond's enemy is the mistress of escapes, so only the chase started in an interesting city gives the possibility of catching her. James wants to arrange his people in such cities. However, if there are not enough interesting cities, the whole action doesn't make sense since Bond's people may wait too long for the enemy.

翻译中…

You are responsible for finding all the interesting cities or saying that there is not enough of them. By not enough, James means strictly less than $20\%$ of all cities.

翻译中…

Input

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The first line contains one integer $t$ ($1 \leq t \leq 2\,000$) — the number of test cases. Each test case is described as follows:

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The first line contains two integers $n$ and $m$ ($1 \leq n \le 10^5$, $0 \leq m \le 2 \cdot 10^5$) — the number of cities and roads between them. Each of the following $m$ lines contains two integers $u$, $v$ ($u \neq v$; $1 \leq u, v \leq n$), which denote that there is a directed road from $u$ to $v$.

翻译中…

You can assume that between each ordered pair of cities there is at most one road. The sum of $n$ over all test cases doesn't exceed $10^5$, and the sum of $m$ doesn't exceed $2 \cdot 10^5$.

翻译中…

Output

翻译中…

If strictly less than $20\%$ of all cities are interesting, print $-1$. Otherwise, let $k$ be the number of interesting cities. Print $k$ distinct integers in increasing order — the indices of interesting cities.

翻译中…

Note

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In all drawings, if a city is colored green, then it is interesting; otherwise, it is colored red.

翻译中…

In the first sample, each city is interesting.

翻译中…

image

翻译中…

In the second sample, no city is interesting.

翻译中…

image

翻译中…

In the third sample, cities $1$, $2$, $3$ and $5$ are interesting.

翻译中…

image

翻译中…

In the last sample, only the city $1$ is interesting. It is strictly less than $20\%$ of all cities, so the answer is $-1$.

翻译中…

image

翻译中…

样例

Input #1
4
3 3
1 2
2 3
3 1
3 6
1 2
2 1
2 3
3 2
1 3
3 1
7 10
1 2
2 3
3 1
1 4
4 5
5 1
4 6
6 7
7 4
6 1
6 8
1 2
2 3
3 4
4 5
5 6
6 1
6 2
5 1
Output #1
1 2 3
-1
1 2 3 5
-1
InputCopy OutputCopy
题目信息
题号
1361E
时间限制
2.5 s
内存限制
256 MB
标签
DFS 及类似
图论
概率论
树

题面与样例版权归 Codeforces 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。