271.Encode_and_Decode_Strings(Medium).md

271. Encode and Decode Strings (Medium)

Date and Time: Sep 14, 2024, 18:45 (EST)

Link: https://leetcode.com/problems/encode-and-decode-strings/

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.

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?

Walk-through:

encode:

We can append each word with a starting len(word) + "#" + word. E.g. (4#abcd).

decode:

When we loop over s, we can use i to find the length and j to find "#", after we find these, we can get the length = int(s[i:j]). Finally, we can append s[j+1: j+1+length] to get the word from s, and we update i = j+1+length.

Python Solution:

class Codec:
    def encode(self, strs: List[str]) -> str:
        """Encodes a list of strings to a single string.
        """
        res = ""
        for s in strs:
            res += str(len(s)) + '#' + s
        return res

    def decode(self, s: str) -> List[str]:
        """Decodes a single string to a list of strings.
        """
        res, i = [], 0
        while i < len(s):
            j = i
            while s[j] != '#':
                j += 1
            length = int(s[i:j])
            res.append(s[j+1: j+1+length])
            i = j + 1 + length
        return res

Time Complexity: $O(n)$, n is the length of all chars in strs.
Space Complexity: $O(n)$