hello-algo/en/codes/ruby/chapter_searching/two_sum.rb

=begin
File: two_sum.rb
Created Time: 2024-04-09
Author: Blue Bean (lonnnnnnner@gmail.com)
=end

### Method 1: brute-force enumeration ###
def two_sum_brute_force(nums, target)
  # Two nested loops give a time complexity of O(n^2)
  for i in 0...(nums.length - 1)
    for j in (i + 1)...nums.length
      return [i, j] if nums[i] + nums[j] == target
    end
  end

  []
end

### Method 2: auxiliary hash table ###
def two_sum_hash_table(nums, target)
  # The auxiliary hash table uses O(n) space
  dic = {}
  # A single loop gives a time complexity of O(n)
  for i in 0...nums.length
    return [dic[target - nums[i]], i] if dic.has_key?(target - nums[i])

    dic[nums[i]] = i
  end

  []
end

### Driver code ###
if __FILE__ == $0
  # ======= Test case =======
  nums = [2, 7, 11, 15]
  target = 13

  # ====== Driver code ======
  # Method 1
  res = two_sum_brute_force(nums, target)
  puts "Method 1: res = #{res}"
  # Method 2
  res = two_sum_hash_table(nums, target)
  puts "Method 2: res = #{res}"
end