python二叉树遍历代码if输出表示语句

以下是Python中二叉树的遍历代码，包括前序遍历、中序遍历和后序遍历，并附有if输出表示语句：

# 定义二叉树节点类
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

# 前序遍历
def preorderTraversal(root: TreeNode) -> List[int]:
    if not root:
        return []
    res = []
    stack = [root]
    while stack:
        node = stack.pop()
        res.append(node.val)
        if node.right:
            stack.append(node.right)
        if node.left:
            stack.append(node.left)
    return res

# 中序遍历
def inorderTraversal(root: TreeNode) -> List[int]:
    if not root:
        return []
    res = []
    stack = []
    cur = root
    while cur or stack:
        while cur:
            stack.append(cur)
            cur = cur.left
        node = stack.pop()
        res.append(node.val)
        cur = node.right
    return res

# 后序遍历
def postorderTraversal(root: TreeNode) -> List[int]:
    if not root:
        return []
    res = []
    stack = [root]
    while stack:
        node = stack.pop()
        res.append(node.val)
        if node.left:
            stack.append(node.left)
        if node.right:
            stack.append(node.right)
    return res[::-1]

其中，if输出表示语句可以在需要输出节点值的地方添加，例如：

def inorderTraversal(root: TreeNode) -> List[int]:
    if not root:
        return []
    res = []
    stack = []
    cur = root
    while cur or stack:
        while cur:
            stack.append(cur)
            cur = cur.left
        node = stack.pop()
        if node.val % 2 == 0:  # 如果节点值为偶数，输出节点值
            res.append(node.val)
        cur = node.right
    return res