每日算法题 Arithmetic Binary Tree

Hi, here's your problem today. This problem was recently asked by Apple:

You are given a binary tree representation of an arithmetic expression. In this tree, each leaf is an integer value,, and a non-leaf node is one of the four operations: '+', '-', '*', or '/'.

Write a function that takes this tree and evaluates the expression.

Example:

    *

   / \

  +    +

 / \  / \

3  2  4  5

This is a representation of the expression (3 + 2) * (4 + 5), and should return 45.

Here's a starting point:

class Node:

  def __init__(self, val, left=None, right=None):

    self.val = val

    self.left = left

    self.right = right



PLUS = "+"

MINUS = "-"

TIMES = "*"

DIVIDE = "/"



def evaluate(root):

  # Fill this in.



tree = Node(TIMES)

tree.left = Node(PLUS)

tree.left.left = Node(3)

tree.left.right = Node(2)

tree.right = Node(PLUS)

tree.right.left = Node(4)

tree.right.right = Node(5)

print evaluate(tree)

# 45