message: Update second-minimum-node-in-a-binary-tree.py
diff: # Time: O(n)
  # Space: O(h)
  +# Given a non-empty special binary tree consisting of nodes with the non-negative value,
  +# where each node in this tree has exactly two or zero sub-node. If the node has two sub-nodes,
  +# then this node's value is the smaller value among its two sub-nodes.
  +#
  +# Given such a binary tree, you need to output the second minimum value in the set made of
  +# all the nodes' value in the whole tree.
  +#
  +# If no such second minimum value exists, output -1 instead.
  +#
  +# Example 1:
  +# Input:
  +# 2
  +# / \
  +# 2 5
  +# / \
  +# 5 7
  +#
  +# Output: 5
  +# Explanation: The smallest value is 2, the second smallest value is 5.
  +# Example 2:
  +# Input:
  +# 2
  +# / \
  +# 2 2
  +#
  +# Output: -1
  +# Explanation: The smallest value is 2, but there isn't any second smallest value.
  +
  # Definition for a binary tree node.
  # class TreeNode(object):
  # def __init__(self, x):
  