Delete Characters to Make Fancy String

This is an easy level leetcode problem, which you can use stack to solve.

Please refer to this link for more detail of the problem.

Essentially, you cannot accept 3 or more consecutive duplicate letters. There could be many ways but using a stack seems to be the most elegant approach. Basically, you accumulate each letter if it doesn’t match with the top two entries in the stack or stack has less than 2 elements. This way, you are only accumulating letters that are not duplicate of 3 or more consecutive characters.

class Solution:
    def makeFancyString(self, s: str) -> str:
        fancy = []
        for letter in s:
            if len(fancy) < 2 or fancy[-1] != letter or fancy[-2] != letter:
                fancy.append(letter)
            
        return "".join(fancy)