如何在二叉树中插入字符？

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

def insertNode(root, val):
    if root is None:
        return TreeNode(val)
    
    if val < root.val:
        root.left = insertNode(root.left, val)
    else:
        root.right = insertNode(root.right, val)
    
    return root

# 示例用法
root = None
root = insertNode(root, 'C')
root = insertNode(root, 'A')
root = insertNode(root, 'B')
root = insertNode(root, 'E')
root = insertNode(root, 'D')

# 打印二叉树中序遍历结果
def inorderTraversal(root):
    if root:
        inorderTraversal(root.left)
        print(root.val)
        inorderTraversal(root.right)

inorderTraversal(root)