271. 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.

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?

Solutions

Solution 1: Encode String Length

During encoding, we convert the length of the string into a fixed 4-digit string, add the string itself, and append it to the result string in sequence.

During decoding, we first take the first four digits of the string to get the length, and then cut the following string according to the length. We cut it in sequence until we get the list of strings.

The time complexity is $O(n)$.

Python3JavaC++Go

class Codec:
    def encode(self, strs: List[str]) -> str:
        """Encodes a list of strings to a single string."""
        ans = []
        for s in strs:
            ans.append('{:4}'.format(len(s)) + s)
        return ''.join(ans)

    def decode(self, s: str) -> List[str]:
        """Decodes a single string to a list of strings."""
        ans = []
        i, n = 0, len(s)
        while i < n:
            size = int(s[i : i + 4])
            i += 4
            ans.append(s[i : i + size])
            i += size
        return ans


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

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        StringBuilder ans = new StringBuilder();
        for (String s : strs) {
            ans.append((char) s.length()).append(s);
        }
        return ans.toString();
    }

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

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

class Codec {
public:
    // Encodes a list of strings to a single string.
    string encode(vector<string>& strs) {
        string ans;
        for (string s : strs) {
            int size = s.size();
            ans += string((const char*) &size, sizeof(size));
            ans += s;
        }
        return ans;
    }

    // Decodes a single string to a list of strings.
    vector<string> decode(string s) {
        vector<string> ans;
        int i = 0, n = s.size();
        int size = 0;
        while (i < n) {
            memcpy(&size, s.data() + i, sizeof(size));
            i += sizeof(size);
            ans.push_back(s.substr(i, size));
            i += size;
        }
        return ans;
    }
};

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

type Codec struct {
}

// Encodes a list of strings to a single string.
func (codec *Codec) Encode(strs []string) string {
    ans := &bytes.Buffer{}
    for _, s := range strs {
        t := fmt.Sprintf("%04d", len(s))
        ans.WriteString(t)
        ans.WriteString(s)
    }
    return ans.String()
}

// Decodes a single string to a list of strings.
func (codec *Codec) Decode(strs string) []string {
    ans := []string{}
    i, n := 0, len(strs)
    for i < n {
        t := strs[i : i+4]
        i += 4
        size, _ := strconv.Atoi(t)
        ans = append(ans, strs[i:i+size])
        i += size
    }
    return ans
}

// Your Codec object will be instantiated and called as such:
// var codec Codec
// codec.Decode(codec.Encode(strs));

271. Encode and Decode Strings 🔒

Description

Solutions

Solution 1: Encode String Length

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