From http://routetomastery.com/blog/2017/01/08/has-pair-with-some-problem/

has_pair.rb

def has_pair_with_sum_brute_force(input, sum)
  # Boolean coercion. Just returning an int (index where found) can be confusing
  !!input.find.with_index do |value, i|
    # All we need to do is keep the inner loop one ahead of the outer.
    input[(i + 1)..-1].find do |sub_value|
      value + sub_value == sum
    end
  end
end

def has_pair_with_sum_in_ordered_list(input, sum)
  # We don't really care about a single item from input here, just positions
  input.each_with_object([0, -1]) do |_, position|
    item_sum = input[position[0]] + input[position[1]]

    # We found a match
    return true if item_sum == sum
    # The pointers collided
    return false if position[0] == input.size + position[1]

    position[0] += 1 if item_sum < sum
    position[1] -= 1 if item_sum > sum
  end

  false
end

def has_pair_with_sum_in_unordered_list(input, sum)
  input.each_with_object(Set.new) do |value, complements|
    return true if complements.include?(value)
    
    complements.add(sum - value)
  end

  false
end