two_sum.rb

require 'rspec/autorun'
require 'pry'

def two_sum(nums, target)
  # for each integer in the input list
  # calculate the sum with every other member
  nums.each_with_index do |outer_n, outer_index|
    nums.detect.with_index do |inner_n, inner_index|
      next if inner_index == outer_index
      return [outer_index, inner_index] if outer_n + inner_n == target
    end
  end
end

RSpec.describe "two_sum" do
  it 'returns the indexes of the two numbers which sum to the target' do
    nums = [2, 7, 11, 15]
    target = 9
    expect(two_sum(nums, target)).to eq [0, 1]
  end

  it 'does not use an element twice' do
    nums = [3, 2, 4]
    target = 6
    expect(two_sum(nums, target)).to eq [1, 2]
  end

  it 'works with negative numbers' do
    nums = [3, 0, 0, 4, -3]
    target = 0
    expect(two_sum(nums, target)).to eq [0, 4]
  end
end