Daily Interview Problem: Reconstrunct Binary Tree from Preorder and Inorder Traversals

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

You are given the preorder and inorder traversals of a binary tree in the form of arrays. Write a function that reconstructs the tree represented by these traversals.

Example:
Preorder: [a, b, d, e, c, f, g]
Inorder: [d, b, e, a, f, c, g]

The tree that should be constructed from these traversals is:

    a

   / \

  b   c

 / \ / \

d  e f  g

Here's a start:

from collections import deque



class Node(object):

  def __init__(self, val):

    self.val = val

    self.left = None

    self.right = None



  def __str__(self):

    q = deque()

    q.append(self)

    result = ''

    while len(q):

      n = q.popleft()

      result += n.val

      if n.left:

        q.append(n.left)

      if n.right:

        q.append(n.right)



    return result





def reconstruct(preorder, inorder):

  # Fill this in.



tree = reconstruct(['a', 'b', 'd', 'e', 'c', 'f', 'g'],

                   ['d', 'b', 'e', 'a', 'f', 'c', 'g'])

print tree

# abcdefg