Problem Statement:

Given a palindromic string of lowercase English letters palindrome, replace exactly one character with any lowercase English letter so that the resulting string is not a palindrome and that it is the lexicographically smallest one possible.

Return the resulting string. If there is no way to replace a character to make it not a palindrome, return an empty string.

A string a is lexicographically smaller than a string b (of the same length) if in the first position where a and b differ, a has a character strictly smaller than the corresponding character in b. For example, "abcc" is lexicographically smaller than "abcd" because the first position they differ is at the fourth character, and 'c' is smaller than 'd'.

Example 1:

Input: palindrome = "abccba"
Output: "aaccba"
Explanation: There are many ways to make "abccba" not a palindrome, such as "zbccba", "aaccba", and "abacba".
Of all the ways, "aaccba" is the lexicographically smallest.

Example 2:

Input: palindrome = "a"
Output: ""
Explanation: There is no way to replace a single character to make "a" not a palindrome, so return an empty string.

Constraints:

1 <= palindrome.length <= 1000
palindrome consists of only lowercase English letters.

Solution:

The idea is to walk till centre and stop if you do not see ‘a’. If you have reached centre than the input was either a string of type aaxaa (odd len(input) and centre is not a) or aaa..a (only a’s). In both cases, the solution is to replace last character with a ‘b’.

 
class Solution {
public:
    string breakPalindrome(string palindrome) 
    {
        int n = palindrome.length(), i=0;
        if (n==1) return "";
        while (i<n/2 && palindrome[i]=='a') i++;
        if (i==n/2) palindrome[n-1] = 'b';
        else palindrome[i] = 'a';
        return palindrome;
    }
};