Suppose an arithmetic expression is given as a binary tree. Each leaf is an integer and each internal node is one of '+', '−', '∗', or '/'. Given the root to such a tree, write a function to evaluate it. For example, given the following tree: * / \ + + / \ / \ 3 2 4 5 You should return 45, as it is (3 + 2) * (4 + 5).

daily-coding-problem-50.rb

#!/usr/bin/env ruby

class Node
    attr_accessor :val, :left, :right
    def initialize val, left=nil, right=nil 
        @val = val 
        @left = left 
        @right = right
    end
end 

def in_order(node)
    str = ""
    if not node.nil?
        str += "(" if not node.left.nil?
        str += in_order(node.left)
        str += node.val
        str += in_order(node.right)
        str += ")" if not node.right.nil?
    end
    return str
end

left = Node.new('+', Node.new('3'), Node.new('2'))
right = Node.new('+', Node.new('4'), Node.new('5'))
root = Node.new('*', left, right)
puts eval(in_order(root))