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## Hackerrank - Journey to the Moon Solution

Beeze Aal

The member states of the UN are planning to send  people to the moon. They want them to be from different countries. You will be given a list of pairs of astronaut ID's. Each pair is made of astronauts from the same country. Determine how many pairs of astronauts from different countries they can choose from.

For example, we have the following data on 2 pairs of astronauts, and 4 astronauts total, numbered  through .

1   2
2   3


Astronauts by country are  and . There are  pairs to choose from:  and .

Function Description

Complete the journeyToMoon function in the editor below. It should return an integer that represents the number of valid pairs that can be formed.

journeyToMoon has the following parameter(s):

• n: an integer that denotes the number of astronauts
• astronaut: a 2D array where each element  is a  element integer array that represents the ID's of two astronauts from the same country

Input Format

The first line contains two integers  and , the number of astronauts and the number of pairs.
Each of the next  lines contains  space-separated integers denoting astronaut ID's of two who share the same nationality.

Constraints

Output Format

An integer that denotes the number of ways to choose a pair of astronauts from different countries.

Sample Input 0

5 3
0 1
2 3
0 4

Sample Output 0

6

Explanation 0

Persons numbered  belong to one country, and those numbered  belong to another. The UN has  ways of choosing a pair:

Sample Input 1

4 1
0 2

Sample Output 1

5

Explanation 1

Persons numbered  belong to the same country, but persons  and  don't share countries with anyone else. The UN has  ways of choosing a pair:

### Solution in Python

from collections import defaultdict

def group_team(astronaut):
country = defaultdict(set)
for i,(a,b) in enumerate(astronaut):
country[i].update([a,b])
while country:
i = list(country.keys())
x = country[i]
pops = [k for k,v in country.items() if v & x]
if len(pops) ==1:
country.pop(i)
yield x
else:
for p in pops:
x = x | country.pop(p)
country[i] = x

def journeyToMoon(n, astronaut):
arr = [len(i) for i in group_team(astronaut)]
s = sum(arr)
c = 0
for i in range(s,n):
arr.append(1)
s+=1
for i in arr:
s-=i
c+= i*s
return c

n,p = map(int,input().split())
astronaut = [list(map(int,input().split())) for _ in range(p)]
print(journeyToMoon(n, astronaut))

First we create a helper function group_team which associates astronauts to same group if they have any matching values.

For example [0,1] , [2,3], [0,4] will return

[{0,1,4}, {2,3}]

Now in next step we will convert this list values to count of items inside the values and our list will become.

[3,2]

Let's assume  n = 8

As we can see that we have only total of 5 astronauts instead of 8 we will assume the 3 astronauts which isn't included in our list as a team of individuals.

Now our list of teams will have the following count of astronauts in each team.

[3,2,1,1,1]

Now we just have to move from left to right.

Team 1 has 3 astronauts, and each of those astronauts can be paired with astronauts in other teams [2,1,1,1]

There can be 3*(2+1+1+1) = 3*5 = 15 different ways to do so

We move to Team 2.

Team 2 has 2 astronauts, and each of those astronauts can be paired with astronauts in other teams [1,1,1]

There can be 2*(1+1+1) = 2*3= 6 different ways to do so

Since we have already calculated all the possible pairings with team 1. We will no longer include team 1(items in the left).

We move to Team 3.

Team 3 has 1 astronauts, and each of those astronauts can be paired with astronauts in other teams [1,1]

There can be 1*(1+1) = 1*2= 2 different ways to do so

Team 4 and Team 5 can together make a pair of 1.

Now we just have to add all these values and we will get the total number of possibilities 15+6+2+1= 24