271. Encode and Decode Strings

/*
Encode function
Time Complexity: O(Number of strings in the list)
Space Complexity: O(Sum of length of all strings in the list)

Decode function
Time Complexity: O(Length of the string)
Space Complexity: O(1)
*/
public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        if (strs == null || strs.size() == 0) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        for (String str : strs) {
            sb.append(str.length());
            sb.append("/");
            sb.append(str);
        }
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> result = new ArrayList();
        if (s == null || s.length() == 0) {
            return result;
        }
        int i = 0;
        while (i < s.length()) {
            int slashIndex = s.indexOf("/", i);
            int strLength = Integer.valueOf(s.substring(i, slashIndex));
            result.add(s.substring(slashIndex+1, slashIndex+strLength+1));
            i = slashIndex+strLength+1;
        }
        return result;
    }
}

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