equi.rb

# http://blog.codility.com/2011/03/solutions-for-task-equi.html
# O(n) space/time complexity answer

# def solution(a)
#     return -1 if a.length == 0
#     total_sum = a.inject(&:+)
#     left_sum = 0
#     a.each_with_index do |element, i|
#         right_sum = total_sum - element - left_sum
#         return i if left_sum == right_sum
#         left_sum += element
#     end
#     -1
# end

# https://codesays.com/2014/solution-to-equi-by-codility/
def solution(a)
  left = 0
  right = a.sum
  a.each_with_index do |element, index|
    right -= element
    return index if left == right
    left += element
  end
  -1
end

a = [-1, 3, -4, 5, 1, -6, 2, 1]
p solution(a)