##### Canadian Computing Competition: 2013 Stage 1, Senior #4

You have a few minutes before your class starts, and you decide to compare the heights of your classmates. You don't have an accurate measuring device, so you just compare relative heights between two people: you stand two people back-to-back, and determine which one of the two is taller. Conveniently, none of your classmates are the same height, and you always compare correctly (i.e., you never make a mistake in your comparisons).

After you have done all of your comparisons, you would like to determine who the tallest person is between two particular classmates.

#### Input Specification

The first line contains two integers and separated by one space. , the number of people in the class, is an integer with . , the number of comparisons that have already been done, is an integer with . Each of the next lines contains two distinct integers and () separated by a space, indicating that person number was determined to be taller than person number . Finally, the last line contains two distinct integers and () separated by one space: your goal is to determine, if possible, whether person is taller than person . Note that it may be the case that neither nor occur in the input concerning measurements between classmates, and each measurement between any two particular people will be recorded exactly once.

#### Output Specification

The output is one line, containing one of three possible strings:

- yes (if is taller than ),
- no (if is taller than ),
- unknown (if there is not enough information to determine the relative heights of and ).

#### Sample Input 1

```
10 3
8 4
3 8
4 2
3 2
```

#### Output for Sample Input 1

`yes`

#### Sample Input 2

```
10 3
3 8
2 8
3 4
3 2
```

#### Output for Sample Input 2

`unknown`

## Comments

DFS Java doesn't work but BFS does. Interesting.

I am short.

BFS not good enough?I implemented a rather simple BFS for this question, but I MLE'd test case 6-1 in PyPy2 and TLE'd it in normal python. Is this meant to be a BFS question?

It is solvable in Python, but maybe try C or Java?

I'm not very familiar with other languages at the moment, so I would just like to know if the other python solutions used BFS or a different approach (or a different twist on BFS).

Most solutions used bfs or dfs. Python 3 and PyPy3 seem to be a bit more lenient on memory than Python 2 and PyPy2 for this question (for me at least).

Ok I figured it out, the issue lay in the fact that I used sys.stdin.read().split('\n') to read the whole file, and since the file was huge, it took up a lot of memory. I just switched to redefining 'input()' to sys.stdin.readline. Thanks for the help!

It most certainly is.

I really like the fact that this problem goes for the realism. Its something that you rarely see in a programming contest. I appreciate CCC for acknowledging the fact that class sizes in Canada have become far too large. Join this petition to create classrooms where LESS than 1 000 000 have to be crammed together. I mean, how would it be even remotely possible to personally teach all of these children. Think of the children!

std::sort(students, students + numStudents);

I wish c++ worked in real life

memset(students_marks, 0, numstudents);