write a brute force solution which gives correct answer and test your current solution by generation random numbers using generator and find the test case where your solution result in wrong answer

Understand what each section is doing. Then Just check line by line. Think about whether it's reasonable carefully. Pay special attention to the interaction between operations.

In addition, you can write down error-prone things before you write code.

You can read um_nik's blog. I think it's meticulous and useful.