16Spring Berkeley cs61b lab5的疑问

如题，lab5是一个简单的editor。其中Task3，插入一段代码，是的可以删除，就是按delete 正常删除。小弟新手，求老司机指点Task 3: Backspace

Now modify your code so that backspace properly removes the most recently typed character. Once you're done with this, you're done with the lab.

链接如下： http://datastructur.es/sp16/materials/lab/lab5/lab5.html