text: #!/usr/bin/python3
  """
  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:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  
  class Solution:
      def findSecondMinimumValue(self, root: TreeNode) -> int:
          ret = self.find(root)
          return -1 if ret == float('inf') else ret
  
      def find(self, root: TreeNode) -> int:
          """
          find the second min
          """
          if not root:
              return float('inf')
  
          if root.left and root.right:
              if root.left.val == root.val:
                  left = self.find(root.left)
              else:
                  left = root.left.val
  
              if root.right.val == root.val:
                  right = self.find(root.right)
              else:
                  right = root.right.val
  
              return min(left, right)
  
          return float('inf')
  