Minimum Deletions to Make Character Frequencies Unique

class Solution {
    public int minDeletions(String s) {
        HashMap<Character, Integer> map= new HashMap<>();
        for(char ch: s.toCharArray()){
            if(map.containsKey(ch)) map.put(ch, map.get(ch)+1);
            else map.put(ch, 1);
        }
        int res=0;
        HashMap<Integer, Boolean> oMap= new HashMap<>();
        for(int occ: map.values()){
            if(oMap.containsKey(occ)){
                while(occ-1>=0){
                    if(!oMap.containsKey(occ-1)){
                        oMap.put(occ-1, true);
                        res++;
                        break;
                    }else{
                        occ--;
                        res++;
                    }
                }
            }
            else oMap.put(occ, true);
        }
        return res;
    }
}