在二叉树中寻找次小元素

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def findSecondMinimumValue(root):
    minValue = float('inf')
    secondMinValue = float('inf')
    
    def dfs(node):
        nonlocal minValue, secondMinValue
        
        if not node:
            return
        
        if node.val < minValue:
            minValue = node.val
        elif minValue < node.val < secondMinValue:
            secondMinValue = node.val
        
        dfs(node.left)
        dfs(node.right)
    
    dfs(root)
    
    if secondMinValue == float('inf'):
        return -1
    else:
        return secondMinValue