GuilinDev

Lc0271

05 August 2008

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.

You are not allowed to solve the problem using any serialize methods (such as eval).

Example 1:

Input: dummy_input = ["Hello","World"]
Output: ["Hello","World"]
Explanation:
Machine 1:
Codec encoder = new Codec();
String msg = encoder.encode(strs);
Machine 1 ---msg---> Machine 2

Machine 2:
Codec decoder = new Codec();
String[] strs = decoder.decode(msg);

Example 2:

Input: dummy_input = [""]
Output: [""]

Constraints:

• 1 <= strs.length <= 200
  

• 0 <= strs[i].length <= 200
  

• strs[i]
  

  contains any possible characters out of

  256
  

  valid ASCII characters.

Follow up: Could you write a generalized algorithm to work on any possible set of characters?

分析

Approach 1: Non-ASCII Delimiter

Intuition

Naive solution here is to join strings using delimiters.

What to use as a delimiter? Each string may contain any possible characters out of 256 valid ascii characters.

Seems like one has to use non-ASCII unichar character, for example

unichr(257)

in Python and

Character.toString((char)257)

in Java (it’s character

ā

).

Here it’s convenient to use two different non-ASCII characters, to distinguish between situations of “empty array” and of “array of empty strings”.

Implementation

Use

split

in Java with a second argument

-1

to make it work as

split

in Python.

Complexity Analysis

• Time complexity : O(N) both for encode and decode, where N is a number of strings in the input array.
• Space complexity : O(1) for encode to keep the output, since the output is one string.O(N) for decode keep the output, since the output is an array of strings.

Approach 2: Chunked Transfer Encoding

Pay attention to this approach because last year Google likes to ask that sort of low-level optimisation. Serialize and deserialize BST problem is a similar example.

This approach is based on the encoding used in HTTP v1.1. It doesn’t depend on the set of input characters, and hence is more versatile and effective than Approach 1.

Data stream is divided into chunks. Each chunk is preceded by its size in bytes.

Encoding Algorithm

• Iterate over the array of chunks, i.e. strings.
  • For each chunk compute its length, and convert that length into 4-bytes string.
  • Append to encoded string :
    • 4-bytes string with information about chunk size in bytes.
    • Chunk itself.
• Return encoded string.

Decoding Algorithm

• Iterate over the encoded string with a pointer

  i
  

  initiated as 0. While

  i < n
  

  :
  • Read 4 bytes

    s[i: i + 4]
    

    . It’s chunk size in bytes. Convert this 4-bytes string to integer

    length
    

    .
  • Move the pointer by 4 bytes

    i += 4
    

    .
  • Append to the decoded array string

    s[i: i + length]
    

    .
  • Move the pointer by

    length
    

    bytes

    i += length
    

    .
• Return decoded array of strings.

Complexity Analysis

• Time complexity : O(N) both for encode and decode, where N is a number of strings in the input array.
• Space complexity : O(1) for encode to keep the output, since the output is one string. O(N) for decode keep the output, since the output is an array of strings.

代码

1.

public class Codec {
  // Encodes a list of strings to a single string.
  public String encode(List<String> strs) {
    if (strs.size() == 0) return Character.toString((char)258);

    String d = Character.toString((char)257);
    StringBuilder sb = new StringBuilder();
    for(String s: strs) {
      sb.append(s);
      sb.append(d);
    }
    sb.deleteCharAt(sb.length() - 1);
    return sb.toString();
  }

  // Decodes a single string to a list of strings.
  public List<String> decode(String s) {
    String d = Character.toString((char)258);
    if (s.equals(d)) return new ArrayList();

    d = Character.toString((char)257);
    return Arrays.asList(s.split(d, -1));
  }
}

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

2.

public class Codec {
  // Encodes string length to bytes string
  public String intToString(String s) {
    int x = s.length();
    char[] bytes = new char[4];
    for(int i = 3; i > -1; --i) {
      bytes[3 - i] = (char) (x >> (i * 8) & 0xff);
    }
    return new String(bytes);
  }

  // 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(intToString(s));
      sb.append(s);
    }
    return sb.toString();
  }

  // Decodes bytes string to integer
  public int stringToInt(String bytesStr) {
    int result = 0;
    for(char b : bytesStr.toCharArray())
      result = (result << 8) + (int)b;
    return result;
  }

  // Decodes a single string to a list of strings.
  public List<String> decode(String s) {
    int i = 0, n = s.length();
    List<String> output = new ArrayList();
    while (i < n) {
      int length = stringToInt(s.substring(i, i + 4));
      i += 4;
      output.add(s.substring(i, i + length));
      i += length;
    }
    return output;
  }
}

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