"""
    Pre-order, in-order and post-order traversal of binary trees.

    Author: Wenru Dong
"""
from typing import TypeVar, Generic, Generator, Optional

T = TypeVar("T")

class TreeNode(Generic[T]):
    def __init__(self, value: T):
        self.val = value
        self.left = None
        self.right = None
    

# Pre-order traversal
def pre_order(root: Optional[TreeNode[T]]) -> Generator[T, None, None]:
    if root:
        yield root.val
        yield from pre_order(root.left)
        yield from pre_order(root.right)

# In-order traversal
def in_order(root: Optional[TreeNode[T]]) -> Generator[T, None, None]:
    if root:
        yield from in_order(root.left)
        yield root.val
        yield from in_ord