实现二叉树的前序遍历、中序遍历和后序遍历。

以下是Python实现二叉树的前序遍历、中序遍历和后序遍历的代码示例：

1. 前序遍历

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

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.preorder(root, res)
        return res
    
    def preorder(self, root, res):
        if not root:
            return
        res.append(root.val)
        self.preorder(root.left, res)
        self.preorder(root.right, res)

2. 中序遍历

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.inorder(root, res)
        return res
    
    def inorder(self, root, res):
        if not root:
            return
        self.inorder(root.left, res)
        res.append(root.val)
        self.inorder(root.right, res)

3. 后序遍历

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.postorder(root, res)
        return res
    
    def postorder(self, root, res):
        if not root:
            return
        self.postorder(root.left, res)
        self.postorder(root.right, res)
        res.append(root.val)