Largest Number

public class Solution {
    public String largestNumber(int[] num) {
        if(num == null || num.length == 0) return "";
        StringBuffer sb = new StringBuffer();
        HashMap<Character,List<String>> hm = new HashMap<Character,List<String>>();
        for(int i = 0; i < num.length; i++)
        {
            char c = String.valueOf(num[i]).charAt(0);
            if(hm.containsKey(c))
            {
                hm.get(c).add(String.valueOf(num[i]));
            }
            else
            {
                List<String> list = new ArrayList<String>();
                list.add(String.valueOf(num[i]));
                hm.put(c,list);
            }
        }
        for(char c = ‘9‘; c >= ‘0‘; c--)
        {
            if(hm.containsKey(c))
            {
                List<String> list = hm.get(c);
                Collections.sort(list, new NumberCompare());
                for(int j = 0; j < list.size(); j++)
                {
                    sb.append(list.get(j));
                }
            }
        }
        String rst = sb.toString();
        if(rst.length() != 0 && rst.charAt(0) == ‘0‘) return "0";
        return rst;
        
    }
    private class NumberCompare implements Comparator<String>
    {
        public int compare(String one, String two)
        {
            String first = one + two;
            String second = two + one;
            int len = first.length();
            int i = 0;
            while( i < len)
            {
                char onec = first.charAt(i);
                char twoc = second.charAt(i);
                if(onec == twoc)    i++;
                else if(onec > twoc) return -1;
                else return 1;
            }
            return 0;
        }
    }
}

public class Solution {
    public String largestNumber(int[] num) {
        if (num == null) {
            return null;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        for (int n1: num) {
            list.add(n1);
        }
        
        Collections.sort(list, new Comparator<Integer>(){
            public int compare(Integer o1, Integer o2) {
                String s1 = "" + o1 + o2;
                String s2 = "" + o2 + o1;
                
                return s2.compareTo(s1);
            }
        });
        StringBuilder sb = new StringBuilder();
        for (int n: list) {
            sb.append(n);
        }
        if (sb.charAt(0) == ‘0‘) {
            return "0";
        }  
        return sb.toString();
    }
}