Find the longest common starting substring in a set of strings [closed]

Answer 1

Here's an efficient solution in ruby. I based the idea of the strategy for a hi/lo guessing game where you iteratively zero in on the longest prefix.

Someone correct me if I'm wrong, but I think the complexity is O(n log n), where n is the length of the shortest string and the number of strings is considered a constant.

def common(strings)
  lo = 0
  hi = strings.map(&:length).min - 1
  return '' if hi < lo

  guess, last_guess = lo, hi

  while guess != last_guess
    last_guess = guess
    guess = lo + ((hi - lo) / 2.0).ceil

    if strings.map { |s| s[0..guess] }.uniq.length == 1
      lo = guess
    else
      hi = guess
    end
  end

  strings.map { |s| s[0...guess] }.uniq.length == 1 ? strings.first[0...guess] : ''
end

And some checks that it works:

>> common %w{ interspecies interstelar interstate }
=> "inters"

>> common %w{ dog dalmation }
=> "d"

>> common %w{ asdf qwerty }
=> ""

>> common ['', 'asdf']
=> ""