Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly twoor 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

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):
 
#         self.val = x
#         self.left = None
#         self.right = None
class Solution(object):
    def findSecondMinimumValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        s = set()
        queue = [root]
        while queue and queue[0]:
            node = queue.pop(0)
            s.add(node.val)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
        if len(s) < 2:
            return -1
        else:
            s.remove(min(s))
            return min(s)

671. Second Minimum Node In a Binary Tree 二叉樹中第二小節點