Missing Numbers Hackerrank Solution

MissingNumbers.java

class Result {
    public static List<Integer> missingNumbers(List<Integer> a, List<Integer> b) {
        Map<Integer, Integer> freqA = new HashMap<>();
        Map<Integer, Integer> freqB = new HashMap<>();
        for(int i=0;i<a.size();i++)         freqA.put( a.get(i), freqA.getOrDefault(a.get(i), 0) +1 );
        for(int i=0;i<b.size();i++)         freqB.put( b.get(i), freqB.getOrDefault(b.get(i), 0) +1 );
        List<Integer> missingNumbers = new ArrayList<>();
        for(Integer key : freqB.keySet()){
           int valueInA = freqA.getOrDefault(key, 0), valueInB = freqB.get(key);
            if(valueInA<valueInB){
                missingNumbers.add(key);
            }
        }
        Collections.sort(missingNumbers);
        return missingNumbers;
    }
}