message: Update longest-univalue-path.py
diff: # Time: O(n)
  # Space: O(h)
  +# Given a binary tree, find the length of the longest path
  +# where each node in the path has the same value. This path may or may not pass through the root.
  +#
  +# Note: The length of path between two nodes is represented by the number of edges between them.
  +#
  +# Example 1:
  +# Input:
  +#
  +# 5
  +# / \
  +# 4 5
  +# / \ \
  +# 1 1 5
  +# Output:
  +# 2
  +#
  +# Example 2:
  +# Input:
  +#
  +# 1
  +# / \
  +# 4 5
  +# / \ \
  +# 4 4 5
  +# Output:
  +# 2
  +#
  +# Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.
  +
  # Definition for a binary tree node.
  # class TreeNode(object):
  # def __init__(self, x):
  