Break a Palindrome — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def breakPalindrome(self, palindrome):
        """
        :type palindrome: str
        :rtype: str
        """
        for i in xrange(len(palindrome)//2):
            if palindrome[i] != 'a':
                return palindrome[:i] + 'a' + palindrome[i+1:]
        return palindrome[:-1] + 'b' if len(palindrome) >= 2 else ""