LeetCode Largest Number

Given a list of non negative integers, arrange them such that they form the largest number.

For example, given [3, 30, 34, 5, 9], the largest formed number is 9534330.

Note: The result may be very large, so you need to return a string instead of an integer.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

class StringComparator implements Comparator<String> {

    @Override
    public int compare(String a, String b) {
        if (a.length() == b.length()) {
            return b.compareTo(a);
        } else {
            String ab = a + b;
            String ba = b + a;
            return ba.compareTo(ab);
        }
    }
}
public class Solution {
    public String largestNumber(int[] num) {
        ArrayList<String> list = new ArrayList<String>(num.length);
        for (int i = 0; i < num.length; i++) {
            list.add(String.valueOf(num[i]));
        }
        Collections.sort(list,new StringComparator());
        String result = "";
        for (String s : list) {
            result += s;
        }
        if(result.charAt(0)=='0') return "0";
        return result;
    }
}