equilibrium.rb

def solution(arr)
  index = 0

  while index < arr.count do
    left_s = (index-1) >= 0 ? arr.slice(0..(index-1)).reduce(0, :+) : 0
    right_s = arr.slice((index+1)..(arr.count-1)).reduce(0, :+)

    return index if left_s == right_s

    index +=1
  end

  return -1
end

p solution([1,2,1])