Ruby Coding Interview Question: Most Common Character In The String

Question

Write a method in Ruby that takes a string as input and returns the most common character in the string. If there are multiple characters that occur with the same frequency, return the one that comes first in alphabetical order. The method should be named most_common_character and should take one parameter, str, which represents the input string.

most_common_character("abracadabra") # Output: "a"
most_common_character("hello world") # Output: "l"

Your solution should satisfy the following requirements:

• The method should be case-sensitive, treating uppercase and lowercase characters as distinct.
• If the input string is empty, the method should return nil.

Answer

Here’s a possible solution in Ruby

def most_common_character(str)
  return nil if str.empty?

  char_frequency = Hash.new(0)

  str.each_char do |char|
    char_frequency[char] += 1
  end

  max_frequency = char_frequency.values.max
  most_common_chars = char_frequency.select { |_, freq| freq == max_frequency }.keys
  most_common_chars.min
end

Explanation:

• We first handle the case where the input string is empty by returning nil.