equi.rb

# http://codility.com/ for info

def equi(a)
  pres, posts = 0, a.reduce(:+)

  0.upto(a.length - 1) do |pivot|
    pres += (pivot > 0 ? a[pivot-1] : 0)
    posts -= a[pivot]
    return pivot if pres == posts
  end
  
  -1
end

#puts equi([-7, 1, 5, 2, -4, 3, 0])