#! /usr/local/bin/ruby

def hanoi1(n, a, b, c, &block)
  return if n.zero?
  hanoi1(n-1, a, c, b, &block)
  yield a, b
  hanoi1(n-1, c, b, a, &block)
end

def hanoi(n, &block)
  block_given? or return to_enum(:hanoi, n)
  hanoi1(n, "a", "b", "c", &block)
end

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