I found that for some algorithm questions on leetcode, some answers, that has high performance, use high level builtin functions in the that language. For example, using regular expressions instead of iterating strings, or using a built-in function to solve the question directly.

My questions are:

• what kind of builtin functions/methods we can use?
• Do we need to submit the simplest answers with less code or to implement the basic logic even using high level languages like JavaScript or Python?
• How to analyze the complexity for those builtin functions.