recursion

gistfile1.rb

def rest(ary)
  _,*r = ary
  r
end

# Strategy1 (compute while going down the stack)
def add1(col)
  def add1helper(col,accu)
    if col.empty?
      accu
    else
      add1helper(rest(col),accu + col.first)
    end
  end

  add1helper(col,0)
end

# Strategy 2 (compute while going up the stack)
def add2(col)
  if col.empty?
    0
  else
    col.first + add2(rest(col))
  end
end