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

You are given the root of a binary tree. Invert the binary tree in place. That is, all left children should become right children, and all right children should become left children.

Example:

<pre>
    a
   / \
  b   c
 / \  /
d   e f
</pre>

The inverted version of this tree is as follows:

<pre>
  a
 / \
 c  b
 \  / \
  f e  d
</pre>

Here is the function signature:

<pre lang=py>
class Node:
  def __init__(self, value):
    self.left = None
    self.right = None
    self.value = value
  def preorder(self):
    print self.value,
    if self.left: self.left.preorder()
    if self.right: self.right.preorder()

def invert(node):
  # Fill this in.

root = Node('a') 
root.left = Node('b') 
root.right = Node('c') 
root.left.left = Node('d') 
root.left.right = Node('e') 
root.right.left = Node('f') 

root.preorder()
# a b d e c f 
print "\n"
invert(root)
root.preorder()
# a c f b e d
</pre>