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:

<pre>
    *
   / \
  +    +
 / \  / \
3  2  4  5
</pre>

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

Here's a starting point:

<pre lang=py>
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
</pre>