271. Encode and Decode Strings

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        StringBuilder sb = new StringBuilder();
        for(String str : strs)
        {
            sb.append(str.length()+"#");
            sb.append(str);
        }
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> list = new ArrayList<>();
        while(s.length() > 0)
        {
            int k = 0, i = 0;
            while('0' <= s.charAt(i) && s.charAt(i) <= '9')
            {
                k = k * 10 + s.charAt(i) - '0';
                i++;
            }
            list.add(s.substring(i + 1, i + 1 + k));
            s = s.substring(i + 1 + k);
        }
        return list;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.decode(codec.encode(strs));