Codility-Lesson 2-Challenge 2

codility2-2.rb

def solution(x, a)
  sum=(x*(x+1))/2
  pos=-1
  b=Array.new(x,0)
  (0..a.length-1).each do |i|
    if b[a[i]-1]==0
      b[a[i]-1]=1
      sum-=a[i]
      if sum==0
        pos=i
        break
      end
    end
  end
pos
end