enter the expression string a+b*c
           the expression is correct
           the postfix expression is - abc *+
           enter the value of a-1
           enter the value of b-2
           enter the value of c-3
           the postfix expression is -abc*+
           result= 7

+
             /
          *       a
         /
        b     c

+
  a
  *
    b
    c

Add
+-- a
+-- Mul
    +-- b
    +-- c

Add
+---Sub
|    +---Div
|    |    +---p
|    |    +---q
|    +---y
+---Mul
     +---b
     +---c

# Return the drawn tree as an array of lines.
#
# node ::= string
# node ::= [string, node, node]
def render_tree(node, prefix0 = "", prefix = "")
  if (node.is_a?(String))
    puts prefix0 + node         # Value
  else
    puts prefix0 + node[0]      # Operator
    render_tree(node[1], prefix  + "+---", prefix + "|    ")
    render_tree(node[2], prefix  + "+---", prefix + "     ")
  end
end
render_tree(["Add", ["Sub", ["Div", "p", "q"], "y"], ["Mul", "b", "c"]])