93. Restore IP Addresses

class Solution {
    List<String> res = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        helper(s, new StringBuilder(), 3, 0);
        return res;
    }
    
    private void helper(String s, StringBuilder cur, int dotLeft, int index) {
        if (dotLeft == 0) {
            if (valid(s.substring(index))) {
                cur.append("." + s.substring(index));
                res.add(cur.toString());
            }
            return;
        }
        
        for (int right = index; right < s.length(); right++) {
            if (valid(s.substring(index, right + 1))) {
                int length = cur.length();
                if (dotLeft == 3) {
                    cur.append(s.substring(index, right + 1));
                    helper(s, cur, dotLeft - 1, right + 1);
                    cur.setLength(length);
                } else {
                    cur.append("." + s.substring(index, right + 1));
                    helper(s, cur, dotLeft - 1, right + 1);
                    cur.setLength(length);
                }  
            }
        }
    }
    
    private boolean valid(String s) {
        if (s.length() > 3) return false;
        if (s.length() < 1) return false;
        if (s.charAt(0) == '0' && s.length() > 1) return false;
        if (Integer.valueOf(s) > 255) return false;
        return true;
    }
}