In this **HackerRank Iterables and iterators in python problem solution** 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'.

## HackerRank Iterables and Iterators in python problem solution

import sys; import math; def solve(n, ok, left, found): if (found == True): return 1.0; if(left == 0): return 0.0; ret = 0.0; ret += solve(n - 1, ok - 1, left - 1, True) * ok/n; ret += solve(n - 1, ok, left - 1, found) *(n - ok)/n; return ret; n = int(sys.stdin.readline()); a = sys.stdin.readline().split(); k = int(sys.stdin.readline()); ca = len(filter(lambda x: x == 'a', a)); ret = 0.0; #print n, a, k, ca print solve(n, ca, k, False);

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