1328. Break a Palindrome

class Solution:
    def breakPalindrome(self, s: str) -> str:
        """ tc O(N) sc O(N)
        main idea: loop through half of the string, find the first char not equal to 'a', change it to 'a'
        otherwise:  change last position'char to 'b'
        """
        
        n = len(s)
        if n == 1:
            return ""
        res = list(s)
        for i in range(n//2):
            if s[i] != 'a':
                res[i] = 'a'
                return ''.join(res)
        res[-1] = 'b'
        return ''.join(res)