如何计算给定数字在二叉树中出现的次数？

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

def count_occurrences(root, target):
    if root is None:
        return 0
    count = 0
    if root.value == target:
        count += 1
    count += count_occurrences(root.left, target)
    count += count_occurrences(root.right, target)
    return count

# 构建一个简单的二叉树
#       1
#      / \
#     2   3
#    / \ / \
#   1  2 1  3
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(1)
root.left.right = TreeNode(2)
root.right.left = TreeNode(1)
root.right.right = TreeNode(3)

# 计算数字1出现的次数
print(count_occurrences(root, 1))  # 输出应该是3

def count_occurrences_iterative(root, target):
    if root is None:
        return 0
    stack = [root]
    count = 0
    while stack:
        node = stack.pop()
        if node.value == target:
            count += 1
        if node.right:
            stack.append(node.right)
        if node.left:
            stack.append(node.left)
    return count