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

You are given the root of a binary tree, along with two nodes, A and B. Find and return the lowest common ancestor of A and B. For this problem, you can assume that each node also has a pointer to its parent, along with its left and right child.

<pre lang=py>
class TreeNode:
  def __init__(self, val):
    self.left = None
    self.right = None
    self.parent = None
    self.val = val

def lowestCommonAncestor(root, a, b):
  # Fill this in.

#   a
#  / \
# b   c
#    / \
#   d*  e*
root = TreeNode('a')
root.left = TreeNode('b')
root.left.parent = root
root.right = TreeNode('c')
root.right.parent = root
a = root.right.left = TreeNode('d')
root.right.left.parent = root.right
b = root.right.right = TreeNode('e')
root.right.right.parent = root.right

print lowestCommonAncestor(root, a, b).val
# c
</pre>