public class Solution {
    public String largestNumber(int[] nums) {
        if(nums == null || nums.length == 0)
            return "0";
        String[] str = new String[nums.length];
        for(int i = 0; i < nums.length; i++)
            str[i] = Integer.toString(nums[i]);
        Arrays.sort(str, new desecComp());
        StringBuilder sb = new StringBuilder();
        for(String s : str){
            
            if(sb.toString().length() == 0 && s.equals("0")) // use str.equals(str2) instead of str == str2 will be more stable;
                continue; // omit 0s in the beginning.
            else
                sb.append(s);
        }
        if(sb.toString().equals("")) 
            return "0";
        return sb.toString();
    }

    class desecComp implements Comparator<String> { // do not forget "<String>" and this class is in the whole class
        public int compare(String s1, String s2) { // the return type is int!!! and the function name is compare not class name
            String a = s1 + s2;
            String b = s2 + s1;
            return b.compareTo(a); // descending order so we use b To a
        }    
    } 
}