Checking if a string contains all unique characters
A way to optimize it would be to return as soon as you have found a duplicate, unless the question specifically asks you to list all duplicates.
You could refactor it to something like that:
public static boolean checkUnique(String s) {
    int len = s.length();
    for (int i = 0; i &lt; len; i++) {
        int c = s.codePointAt(i);
        int next = s.indexOf(c, i + 1);
        if (next &gt; i) {
             return false;
        }
    }
    return true;
}

It isn't more efficient as your solution, but I doubt there is an algorithm that can do it better.