271. Encode and Decode String

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 strs) {
  // ... your code
  return encoded_string;

Machine 2 (receiver) has the function:

vector decode(string s) {
  //... your code
  return strs;

So Machine 1 does:

string encoded_string = encode(strs);

and Machine 2 does:

vector strs2 = decode(encoded_string);

strs2 in Machine 2 should be the same as strs in Machine 1.

Implement the encode and decode methods.


  • 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.


We do neet ‘/’ to decode, because we don’t know how long the number is, if the length of a string is 88888, then we need to design a way to get this number, that’s why we use ‘/’ to split length and string.

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        if (strs == null) {
            return null;
        StringBuilder sb = new StringBuilder();
        // Don't forget String
        for (String str : strs) {
            // Don't forget length()
        return sb.toString();

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        // List is abstract, should be instantiated by ArrayList
        List<String> result = new ArrayList<>();
        if (s == null) {
            return result;
        int i = 0;
        while (i < s.length()) {
            // s.indexOf, not String.
            int indexSlash = s.indexOf('/', i);
            // s.substring, not subString
            int length = Integer.valueOf(s.substring(i, indexSlash));
            result.add(s.substring(indexSlash + 1, indexSlash + length + 1));
            i = indexSlash + length + 1;
        return result;

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

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