eq_indices.rb

# An equilibrium index of a sequence is an index into the sequence 
# such that the sum of elements at lower indices is equal to the 
# sum of elements at higher indices.

def eq_indices(arr)
  left, right = 0, arr.inject(0, :+)
  result = []
  arr.each_with_index do |el, i|
    right -= el
    result << i if right == left
    left += el
  end
  result
end

1. Sum up the elements of the array first (O(n)).
2. Loop through the array (O(n))
   • reduce the right value by the value of the current element
   • check if prefix equals to suffix and add the index to the result set if that's the case
   • add current element's value to the value of the prefix

We get a linear total complexity of: O(n) + O(n) => O(n).