271. Encode and Decode Strings

Description

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

Add word length in front, encode O(N * L), decode O(N * L)

public class Codec {

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

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> list = new ArrayList<>();
        int i = 0;
        
        while (i < s.length()) {
            int j = s.indexOf("/", i);  // find the first index of slash from i
            int len = Integer.parseInt(s.substring(i, j));  // find curr word len
            list.add(s.substring(j + 1, j + len + 1));      // read curr word
            i = j + len + 1;
        }
        
        return list;
    }
}

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

Escape, encode O(N * L), decode O(N * L)

Double any hashes inside the strings, then use standalone hashes (surrounded by spaces) to mark string endings. For example:

{"abc", "def"} => "abc # def # "
{'abc', '#def'} => "abc # ##def # "
{'abc##', 'def'} => "abc#### # def # "

For decoding, just do the reverse: First split at standalone hashes, then undo the doubling in each string.

public class Codec {

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

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> list = new ArrayList<>();
        String[] tokens = s.split(" # ");
        
        for (String t : tokens) {
            list.add(t.replace("##", "#"));
        }
        
        return list;
    }
}

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