gistfile1.rb

# Finds equilibrium index of a sequence.
# Returns equilibrium index or -1 if no equilibrium exists.

def equi(arr)
  left  = sums(arr)
  right = sums(arr.reverse)
  for i in 1..(arr.length) do
    return (i-1) if left[i-1] == right[-i]
  end
  -1
end

def sums(arr)
  sums = [0]
  arr.each_with_index do |a, i|
    sums << sums[i] + a unless i == (arr.length - 1)
  end
  sums
end