271. 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.

Solution:

To encode, we use a StringBuilder to concatenate all strings.

For each string, we add its length with a split character '#' at its beginning.

To decode, we go through the serialized sting.

When we find a split character '#', we can parse the length of this string.

Then we add the substring using the length and update the pointers.



Code:

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()).append("#").append(str);
        }
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> strs = new ArrayList<>();
        if (s == null) {
            return strs;
        }
        int i = 0;
        int j = 0;
        while (j < s.length()) {
            if (s.charAt(j) == '#') {
                int count = Integer.parseInt(s.substring(i, j));
                strs.add(s.substring(j + 1, j + 1 + count));
                i = j + 1 + count;
                j = j + 1 + count;
            }
            else {
                j++;
            }
        }
        return strs;
    }
}

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