如何展平df？

import pandas as pd

# 创建一个多维数据框
df = pd.DataFrame({
    'A': [1, 2, 3],
    'B': [[4, 5], [6, 7], [8, 9]],
    'C': [[[10, 11], [12, 13]], [[14, 15], [16, 17]], [[18, 19], [20, 21]]]
})

# 展平df
df_flat = df.stack().reset_index(drop=True)

print(df_flat)

0             1
0    1             4
1    2             5
2    2             6
3    3             7
4    3             8
5    4             9
6    4    [10, 11]
7    5    [12, 13]
8    5    [14, 15]
9    6    [16, 17]
10   6    [18, 19]
11   7    [20, 21]
dtype: object