如何将树递归函数（或算法）转换为循环函数？

def tree_traversal_recursive(root):
    if root is None:
        return
    stack = [root]
    while stack:
        node = stack.pop()
        # 处理当前节点
        print(node.val)
        # 将右节点先压入栈中
        if node.right:
            stack.append(node.right)
        # 将左节点压入栈中
        if node.left:
            stack.append(node.left)

def tree_traversal_recursive(root):
    if root is None:
        return
    queue = [root]
    while queue:
        node = queue.pop(0)
        # 处理当前节点
        print(node.val)
        # 将左节点和右节点依次加入队列
        if node.left:
            queue.append(node.left)
        if node.right:
            queue.append(node.right)