Make the string lexicographically smallest and non palindromic by replacing exactly one character

Question

Given a palindromic string palindrome, replace exactly one character by any lowercase English letter so that the string becomes the lexicographically smallest possible string that isn't a palindrome.

After doing so, return the final string. If there is no way to do so, return the empty string.

I have come up with the following, which takes O(n) time and O(1) space:

def breakPalindrome(self, S):
    for i in xrange(len(S) / 2):
        if S[i] != 'a':
            return S[:i] + 'a' + S[i + 1:]
    return S[:-1] + 'b' if S[:-1] else ''

Is there a better solution? Please correct me if there's some mistake in time or space complexity.

Your time is definitely O(n). You are building a new string, so I could argue that space is also O(n). — Frank Yellin, Oct 31 '20 at 23:59
Forget the iteration; if you're modifying a string at some index, a minimum of O(N) time will be guaranteed — Abhinav Mathur, Nov 01 '20 at 04:53

score 0 · Answer 1 · answered Nov 01 '20 at 05:45

Bug in your code: Odd palindromes. Input 'aba' gives output 'abb'. The correct output should be 'aaa'
Return "" on len(palindrome) == 1 is not a right thing to do. I would treat empty string as a palindrome.
Strings are immutable in python. So, space complexity of your solution is O(n). It can't be any better.
There can't be a better solution in terms of space or time complexity. You have to check the whole string in the worst case e.g. string contains only 'a's.
In languages where strings are mutable e.g. C++, the space complexity could be O(1) as one can change a character at an index in constant time like palindrome[i] = 'a' or palindrome[palindrome.size() - 1] = 'b'.

aaa is a palindrome. – David Eisenstat Nov 01 '20 at 12:23 — David Eisenstat, Nov 01 '20 at 12:23

Make the string lexicographically smallest and non palindromic by replacing exactly one character

1 Answers1