在一个由小写英文字母(a-z)组成的字符串中，查找最长子串，其头尾字母相同，且中间不包含该头尾字母,并输出最左边的该类子串

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();
        String subStr = "";
        for (int i = 0; i < str.length(); i++) {
            String newSubStr = str.charAt(i) + "";
            for (int j = i+1; j < str.length(); j++) {
                if (j == str.length() - 1 && str.charAt(i) != str.charAt(j)){
                    newSubStr = str.charAt(i) + "";
                    break;
                }
                if (str.charAt(i) == str.charAt(j)){
                    newSubStr = str.substring(i,j+1);
                    break;
                }
                newSubStr = str.substring(i,j+1);
            }
            if (newSubStr.length() > subStr.length()){
                subStr = newSubStr;
            }
        }
        System.out.println(subStr);
    }
}