Question

Formatted question description: https://leetcode.ca/all/271.html

271. Encode and Decode Strings

Level

Medium

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

In encode method, store each string’s length so that the strings can be uniquely determined. For each string str in strs, encode it as the following format: str.length() + "/" + str.

  • For example, “encode” is encoded as 6/encode. All strings’ encoding results are concatenated together.

In decode method, each time find the first index of "/", and the substring before "/" is an integer that represents the length of the next string. Convert the substring before "/" into an integer, obtain the substring with the determined length just after "/", and add the substring after "/" to the result list. After one string is decoded, continue with the remaining strings. Finally, return the result list.

You can also use a simpler compression method, add a newline character \0 at the end of each string, which also belongs to a string, so when decoding, just look for the newline character.

Code

Java

import org.apache.commons.io.IOUtils;

import java.io.IOException;
import java.io.StringReader;
import java.util.LinkedList;
import java.util.List;

public class Encode_and_Decode_Strings {

    public class Codec {

        // Encodes a list of strings to a single string.
        public String encode(List<String> strs) {
            String res = "";
            for (String str : strs) res += str + '\0';
            return res;
        }

        // Decodes a single string to a list of strings.
        public List<String> decode(String s) throws IOException {
            return IOUtils.readLines(new StringReader(s));
        }
    }

    public class Codec_2nd_solution {

        // Encodes a list of strings to a single string.
        public String encode(List<String> strs) {
            StringBuffer sb = new StringBuffer();
            int size = strs.size();
            for (int i = 0; i < size; i++) {
                String str = strs.get(i);
                int length = str.length();
                sb.append(length + "/" + str);
            }
            return sb.toString();
        }

        // Decodes a single string to a list of strings.
        public List<String> decode(String s) {
            List<String> list = new ArrayList<String>();
            StringBuffer sb = new StringBuffer(s);

            int endIndex = 0;
            while (endIndex < s.length()) {
                int index = sb.toString().indexOf('/', endIndex); // @note: first `/` after index i
                int length = Integer.parseInt(sb.substring(0, index));
                endIndex = index + length + 1;
                String str = sb.substring(index + 1, endIndex);
                list.add(str);
            }
            return list;
        }
    }

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

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

}

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