NoRepeating.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class NoRepeating {

    public static void main(String[] args) {
        System.out.println(find(null));
        System.out.println(find(""));
        System.out.println(find("111111"));
        System.out.println(find("aaabbbcdeff"));
        System.out.println(find("abcdeefghijjjklmno"));
    }

    public static String find(String str) {
        if (null == str || str == "") return "";
        List<String> result = new ArrayList<>();
        StringBuilder builder = new StringBuilder();
        char[] chars = str.toCharArray();
        char last = 0;
        char current;
        for (int i = 0; i < chars.length; i++) {
            current = chars[i];
            if (current == last) {
                if (builder.length() > 0) {
                    builder.delete(builder.length() - 1, builder.length());
                    if (builder.length() > 0) {
                        result.add(builder.toString());
                        builder.delete(0, builder.length());
                    }
                }
            } else {
                builder.append(chars[i]);
            }
            last = chars[i];
            if (i == chars.length -1 && builder.length() > 0) {
                result.add(builder.toString());
            }
        }
        if (result.size() == 0) return "";
        Collections.sort(result, (a, b) -> b.length() - a.length());
        return result.get(0);
    }
}

Repeating.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Repeating {

    public static void main(String[] args) {
        System.out.println(find(null));
        System.out.println(find(""));
        System.out.println(find("111111"));
        System.out.println(find("aaabbbcdef"));
        System.out.println(find("aaabbbcdeffffff"));
        System.out.println(find("abcdeefghijjjklmno"));
    }

    public static String find(String str) {
        if (null == str || str == "" || str.length() == 1) return str;
        List<String> result = new ArrayList<>();
        StringBuilder builder = new StringBuilder();
        char[] chars = str.toCharArray();
        builder.append(chars[0]);
        for (int i = 1; i < chars.length; i++) {
            if (chars[i] != chars[i-1]) {
                if (builder.length() > 0) {
                    result.add(builder.toString());
                    builder.delete(0, builder.length());
                }
            }
            builder.append(chars[i]);
            if (i == chars.length -1) {
                result.add(builder.toString());
            }
        }
        if (result.size() == 0) return "";
        Collections.sort(result, (a, b) -> b.length() - a.length());
        return result.get(0);
    }
}