671. 二叉树中第二小的节点

class Solution(object):
    def findSecondMinimumValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return -1
        temp = []
        stack = [root]
        while stack:
            sizestack = len(stack)
            for i in range(sizestack):
                node = stack.pop(0)
                temp.append(node.val)
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
        temp.sort()
        return list(set(temp))[1] if len(set(temp)) > 1 else -1