1328. Break a Palindrome

Method 1

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.

Method 1

【O(n) time | O(n) space】

package Leetcode.Greedy;

/**
 * @author zhengxingxing
 * @date 2025/03/05
 */
public class BreakAPalindrome {
    public static String breakPalindrome(String palindrome) {
        // If string length is 1, impossible to break palindrome
        if (palindrome.length() <= 1){
            return "";
        }

        char[] chars = palindrome.toCharArray();
        int n = chars.length;

        // Check first half of string only due to palindrome property
        for (int i = 0; i < n / 2; i++) {
            // If character is not 'a', replace with 'a' for lexicographically smallest result
            if (chars[i] != 'a'){
                chars[i] = 'a';
                return new String(chars);
            }
        }

        // If all characters in first half are 'a', replace last character with 'b'
        chars[n - 1] = 'b';
        return new String(chars);
    }

    public static void main(String[] args) {
        // Test case 1: Regular palindrome
        String test1 = "abccba";
        System.out.println("Test case 1 input: " + test1);
        System.out.println("Test case 1 output: " + breakPalindrome(test1));

        // Test case 2: Single character
        String test2 = "a";
        System.out.println("Test case 2 input: " + test2);
        System.out.println("Test case 2 output: " + breakPalindrome(test2));

        // Test case 3: All 'a' characters
        String test3 = "aaaa";
        System.out.println("Test case 3 input: " + test3);
        System.out.println("Test case 3 output: " + breakPalindrome(test3));
    }
}

Method 1

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