if the notebook cannot load, please refer to https://nbviewer.org/github/zhefeng687/Effective-Pandas/blob/c227b0063e7f2671cffb9716089b32d22f8b8577/Pandas%20Exercises.ipynb or https://app.reviewnb.com/zhefeng687/Effective-Pandas/blob/main/Pandas%20Exercises.ipynb%2F/file