test.rb

list = 10000000.times.map {rand(10)}

def answer(list)
  amount_of_zeros = 0 
  index = list.size - 1
  while index >= 0 
    if list[index].zero?
      amount_of_zeros += 1
    elsif amount_of_zeros > 0
      list[index], list[index+amount_of_zeros] = list[index+amount_of_zeros], list[index]
    end  
    index -= 1
  end
  list
end

def answer2(list)
  list.partition(&:zero?).flatten!
end

require 'benchmark'

p answer(list.dup) == answer2(list.dup)

Benchmark.bmbm do |x|
  x.report("answer1") { answer(list.dup) }
  x.report("answer2")  { answer2(list.dup) }
end