Encode and Decode Strings

https://leetcode.com/problems/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.

Thoughts

题目强烈暗示不能用单一字符作为分割符. 那么想记住string边界, 不难想到可以把长度记下来, 放在分隔符前. 然后每次读出时就把相应长度的substring抽出来放到结果中, index直接跳过它们进入下一个string的长度和分隔符.

注意只有长度没有分隔符也是不行的, 因为不知道数字合适结束.

Code

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<>();
        for (int i = 0; i < s.length(); i++) {
            int slash = s.indexOf('/', i);
            int len = Integer.valueOf(s.substring(i, slash));
            res.add(s.substring(slash + 1, slash + 1 + len));
            i = slash + len;
        }
        return res;
    }
}

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

Analysis

时间复杂度O(N).