Restore IP Addresses

lc 93

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example: Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

    public List<String> restoreIpAddresses(String s) {
        List<String> result = new ArrayList<String>();
        if( s == null || s.length() == 0 || s.length() > 12 || s.length() <4){
            return result;
        }
        List<String> list = new ArrayList<String>();
        helper(result,list, s, 0);
        return result;
    }
    public void helper( List<String> result,List<String> list, String s, int position ){
        if(list.size() == 4){
            if(position != s.length()){
                return;
            }
            StringBuffer sb = new StringBuffer();
            for(String tmp: list){
                sb.append(tmp);
                sb.append(".");
            }
            sb.deleteCharAt(sb.length()-1);
            result.add(sb.toString());
            return;

        }

        for( int i = position; i < s.length() && i <  position + 3;i++){
            String value = s.substring(position, i + 1);
            if(isValid(value)){
                list.add(value);
                helper(result, list, s, i + 1);
                list.remove(list.size() - 1);
            }
        }

    }
    private boolean isValid(String s){
        if(s.charAt(0) == '0')
            return s.equals("0"); // to eliminate cases like "00", "10", ip不能是000，就是能是0
        int digit = Integer.valueOf(s);
        return digit >= 0 && digit <= 255;
    }