对二叉树中的唯一值进行计数

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

def count_unique_values(root):
    value_count = {}
    
    def traverse(node):
        if node is None:
            return
        if node.value in value_count:
            value_count[node.value] += 1
        else:
            value_count[node.value] = 1
        traverse(node.left)
        traverse(node.right)
    
    traverse(root)
    
    unique_count = 0
    for count in value_count.values():
        if count == 1:
            unique_count += 1
    
    return unique_count

# 示例用法
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(2)
root.left.left = TreeNode(3)
root.left.right = TreeNode(4)

print(count_unique_values(root))  # 输出: 3