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.

解法1：

把每一个string里的字符转换成ascii码，用一个＃分隔。同时每一个字符串间用，分隔。
要注意的是，在decode的时候可能会出现,,的情况，所以要用string.split(pattern, -1)来保留所有的empty string

C++

Java

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
       StringBuilder builder = new StringBuilder();
        for (String str : strs) {
            char[] chars = str.toCharArray();
            for (char c : chars) {
                builder.append("" + Integer.toString((int)c));
                builder.append("#");
            }
            builder.append(",");
            // if (!str.length() == 0) {
            //     builder.append("#");
            // }
        }

        return builder.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {

        List<String> res = new ArrayList<String>();
        if (s == null || s.isEmpty()) {
            return res;
        }

        String[] components = s.substring(0, s.length() - 1).split(",",-1);    // Remove the "," at the end of the string
        for (String element : components) {
            StringBuilder builder = new StringBuilder();
            String[] codes = element.split("#");
            for (String code : codes) {
                if (!code.isEmpty()) {
                    builder.append((char)Integer.parseInt(code));
                }
            }
            res.add(builder.length() == 0 ? "" : builder.toString());
        }
        return res;        
    }
}

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

解法2：

把一个string转化为size/stringcontent的形式。

lang: java

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> res = new ArrayList<>();
        int i = 0;
        while (i < s.length()) {
            int pos = s.indexOf('/', i);
            int len = Integer.parseInt(s.substring(i, pos));
            res.add(s.substring(pos + 1, pos + 1 + len));
            i = pos + 1 + len;
        }

        return res;
    }
}

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