Determine if one array contains all elements of another array

Question

I need to tell if an array contains all of the elements of another array with duplicates.

[1,2,3].contains_all? [1,2]   #=> true
[1,2,3].contains_all         


        

Answer 1

This solution will only iterate through both lists once, and hence run in linear time. It might however be too much overhead if the lists are expected to be very small.

  class Array
    def contains_all?(other)
      return false if other.size > size
      elem_counts = other.each_with_object(Hash.new(0)) { |elem,hash| hash[elem] += 1 }
      each do |elem|
        elem_counts.delete(elem) if (elem_counts[elem] -= 1) <= 0
        return true if elem_counts.empty?
      end
      false
    end
  end