add up to 100

insert_signs.rb

def ops(nums, result, sign = 1)
  case nums.size
  when 0, 1
    result == (nums[0] || 0) * sign ? [[nums[0]].compact] : []
  else
    n1, n2 = nums.first(2)
    plus = ops(nums.drop(1), result - n1 * sign, 1).map { |r| [n1, :+] + r }
    minus = ops(nums.drop(1), result - n1 * sign, -1).map { |r| [n1, :-] + r }
    concat = ops([n1 * 10 + n2] + nums.drop(2), result, sign)
    plus + minus + concat
  end
end

ops([1, 2, 3, 4, 5, 6, 7, 8, 9], 100).each { |n| puts n.join }