Day-13 Iterator for Combination

class CombinationIterator {

    int p=0;
    List<String> c=null;
    public CombinationIterator(String characters, int combinationLength) {
        c=new ArrayList<>();
        helper(characters,0,combinationLength,new StringBuilder());
    }
    
    public String next() {
        return c.get(p++);
    }
    
    public boolean hasNext() {
        return p<c.size();
    }
    
    void helper(String s,int pos,int combinationLength,StringBuilder sb){
        if(sb.length()==combinationLength){
            c.add(sb.toString());
            return;
        }
        
        for(int i=pos;i<s.length();i++){
            sb.append(s.charAt(i));
            helper(s,i+1,combinationLength,sb);
            sb.deleteCharAt(sb.length()-1);
        }
    }
}

class CombinationIterator {
    int bitmask, n, k;
    String chars;

    public CombinationIterator(String characters, int combinationLength) {
        n = characters.length();
        k = combinationLength;
        chars = characters;

        bitmask = (1 << n) - (1 << n - k);
    }

    public String next() {
        StringBuilder curr = new StringBuilder();
        for (int j = 0; j < n; j++) {
            if ((bitmask & (1 << n - j - 1)) != 0) {
                curr.append(chars.charAt(j));
            }
        }

        bitmask--;
        while (bitmask > 0 && Integer.bitCount(bitmask) != k) {
            bitmask--;
        }

        return curr.toString();
    }

    public boolean hasNext() {
        return bitmask > 0;
    }
}