#! /usr/local/bin/ruby

def hanoi1(n, a, b, c)
  return if n.zero?
  hanoi1(n-1, a, c, b) { |from, to| yield from, to }
  yield a, b
  hanoi1(n-1, c, b, a) { |from, to| yield from, to }
end

def hanoi(n)
  return to_enum(:hanoi, n) unless block_given?
  hanoi1(n, "a", "b", "c") { |from, to| yield from, to }
end

hanoi(7) { |from, to| puts "#{a} => #{b}" }