leetcode 93 restore ip addresses

class  {
    public List<String> restoreIpAddresses(String s) {
        List<String> ans = new ArrayList<String>();
        StringBuilder ip = new StringBuilder();
        
        for(int A = 1; A <=3; A++){
            for(int B = 1; B <=3; B++){
                for(int C = 1; C <=3; C++){
                    for(int D=1; D <=3; D++){
                        if(A + B + C + D == s.length()){
                            int a = Integer.parseInt(s.substring(0, A));
                            int b = Integer.parseInt(s.substring(A, A+B));
                            int c = Integer.parseInt(s.substring(A+B, A+B+C));
                            int d = Integer.parseInt(s.substring(A+B+C));
                            if(a<=255 && b<=255 && c<=255 && d<= 255){
                                ip.append(a);
                                ip.append('.');
                                ip.append(b);
                                ip.append('.');
                                ip.append(c);
                                ip.append('.');
                                ip.append(d);
                                
                                if(ip.length() == s.length() + 3){
                                    ans.add(ip.toString());
                                }
                                ip.delete(0, ip.length());
                            }
                        }
                    }
                }
            }
        }
        return ans;
    }
}