以下是一个按字母顺序打印二叉树的示例代码：

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
        
def inorderTraversal(root):
    if root is None:
        return []
    
    result = []
    stack = []
    curr = root
    
    while curr or stack:
        while curr:
            stack.append(curr)
            curr = curr.left
        curr = stack.pop()
        result.append(curr.val)
        curr = curr.right
    
    return result

root = TreeNode('F')
root.left = TreeNode('B')
root.right = TreeNode('G')
root.left.left = TreeNode('A')
root.left.right = TreeNode('D')
root.left.right.left = TreeNode('C')
root.left.right.right = TreeNode('E')
root.right.right = TreeNode('I')
root.right.right.left = TreeNode('H')

result = inorderTraversal(root)
print(result)

输出结果为：['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I']