Encode and Decode Strings

Design an algorithm to encode a list of strings to a string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Machine 1 (sender) has the function:
string encode(vector<string> strs) { // ... your code return encoded_string; } 
Machine 2 (receiver) has the function: 
vector<string> decode(string s) { //... your code return strs; }

So Machine 1 does:
string encoded_string = encode(strs); 
and Machine 2 does:
vector<string> strs2 = decode(encoded_string); 
strs2 in Machine 2 should be the same as strs in Machine 1.

Implement the encode and decode methods.

Note: The string may contain any possible characters out of 256 valid ascii characters. Your algorithm should be generalized enough to work on any possible characters. Do not use class member/global/static variables to store states. Your encode and decode algorithms should be stateless. Do not rely on any library method such as eval or serialize methods. You should implement your own encode/decode algorithm.

Not a hard problem. We can use a sentinel character to concatenate strings. The tricky part is that the string can have any characters, including our sentinel. We can also track the length of the string, so the structure can be len_string + sentinel_char + len_string + sentinel_char.

public String encode(List strs) {
        StringBuilder sb = new StringBuilder();
        for (String s : strs) {
            sb.append(s.length()).append('$').append(s).append('$');
        }
        return sb.toString();
    }

    public List decode(String s) {
        List rst = new ArrayList<>();
        int index = 0;
        while (index < s.length()) {
            int pos = index;
            while (pos < s.length() && Character.isDigit(s.charAt(pos))) {
                pos++;
            }
            int len = Integer.valueOf(s.substring(index, pos));
            if (s.charAt(pos++) != '$') {
                throw new InvalidParameterException("Invalid input string!");
            }
            rst.add(s.substring(pos, pos + len));
            pos += len;
            if (s.charAt(pos++) != '$') {
                throw new InvalidParameterException("Invalid input string!");
            }
            index = pos;
        }
        return rst;
    }