HackerRank Python Solution - Itertools Topic - iterables and iterators

  • The itertools module standardizes a core set of fast, memory-efficient tools that are useful by themselves or in combination. Together, they form an iterator algebra making it possible to construct specialized tools succinctly and efficiently in pure Python.
  • You are given a list of N lowercase English letters. For a given integer K, you can select any K indices (assume 1-based indexing) with a uniform probability from the list.
  • Find the probability that at least one of the K indices selected will contain the letter: 'a'.
Input Format:
  • The input consists of three lines. The first line contains the integer N, denoting the length of the list. The next line consists of N space-separated lowercase English letters, denoting the elements of the list.
  • The third and the last line of input contains the integer K, denoting the number of indices to be selected. 
Output Format:

Output a single line consisting of the probability that at least one of the K indices selected contains the letter:'a'.

Note: The answer must be correct up to 3 decimal places.

Constraints:
  • 1 <= N <= 10
  • 1 <= K <= N
All the letters in the list are lowercase English letters.

Sample Input:

4 
a a c d
2
Sample Output:

0.8333
Explanation:
  • All possible unordered tuples of length comprising of indices from 1 to 4 are: (1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)
  • Out of these 6 combinations, 5 of them contain either index 1 or index 2 which are the indices that contain the letter 'a'.
  • Hence, the answer is 5/6.
Solution:

from itertools import combinations

N = int(input())
Word = input().split()
K = int(input())

count = 0
for item in combinations(Word,K):
    if 'a' in item:
        count+=1
        
print(count/len(list(combinations(Word,K))))
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