Encode and Decode Strings 解答

Question

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 1 -- JSON format

第一种方法是参考的JSON的规则。

Encode: 我们将输入的字符串数组封装成JSON中的array。[ (left bracket) and ] (right bracket) 表示开头的结尾。中间的分隔用, (comma)。然后对于每个字符串，是 wrapped in double quotes。

由于字符串中本来就可能有双引号或是back slash ()，所以我们需要对这两种符号做转义。方法是多加一个back slash

如 原字符串 "aafg"  -> \"aafg"

JSON里还有更复杂的字符串处理方法。但我们这里的目标只是让encode，再decode后的字符串相同，所以不必那么复杂。

Decode处理原则如下

1. 一个boolean的variable记录当前应该是下一个字符串的开头还是当前字符串的结束

2. 碰到bracket，根据是开始／结束，新建一个空字符串／将当前的字符串存入结果中

3. 碰到back slash，看它下一个元素是否是back slash / bracket，如果是，则将它下一个元素加到字符串中，计数加一。

public class Codec {
    private final char start = '[';
    private final char end = ']';
    private final char include = '"';
    private final char strSplit = ',';

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        StringBuilder sb = new StringBuilder();
        sb.append(start);
        for (String str : strs) {
            sb.append(include);
            int len = str.length();
            for (int i = 0; i < len; i++) {
                char current = str.charAt(i);
                if (current == '"' || current == '\') {
                    sb.append('\');
                }
                sb.append(current);
            }
            sb.append(include);
            sb.append(strSplit);
        }
        sb.append(end);
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> result = new ArrayList<String>();
        if (s == null || s.length() < 1) {
            return result;
        }
        int len = s.length();
        if (s.charAt(0) != start || s.charAt(len - 1) != end) {
            return result;
        }
        boolean startSymbol = true;
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i < len - 1; i++) {
            char current = s.charAt(i);
            if (current == include) {
                if (startSymbol) {
                    sb = new StringBuilder();
                } else {
                    result.add(sb.toString());
                }
                startSymbol = !startSymbol;
                continue;
            }
            if (current == strSplit && startSymbol) {
                continue;
            }
            if (current == '\') {
                char next = s.charAt(i + 1);
                if (next == '\' || next == '"') {
                    sb.append(next);
                    i++;
                    continue;
                }
            }
            sb.append(current);
        }
        return result;
    }
}

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

Solution 2 

利用了Java里String的 int indexOf(int ch, int fromIndex)函数。

同时存入字符串和字符串的长度。

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> result = new ArrayList<String>();
        int length = s.length();
        int i = 0;
        while (i < length) {
            int slash = s.indexOf('/', i);
            int size = Integer.valueOf(s.substring(i, slash));
            result.add(s.substring(slash + 1, slash + size + 1));
            i = slash + size + 1;
        }
        return result;
    }
}