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Question

Formatted question description: https://leetcode.ca/all/271.html

271. Encode and Decode Strings

Level

Medium

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.

Solution

In encode() method, store each string’s length so that the strings can be uniquely determined. For each string str in strs, encode it as the following format: str.length() + "/" + str.

  • For example, “encode” is encoded as 6/encode. All strings’ encoding results are concatenated together.

In decode() method, each time find the first index of "/", and the substring before "/" is an integer that represents the length of the next string. Convert the substring before "/" into an integer, obtain the substring with the determined length just after "/", and add the substring after "/" to the result list. After one string is decoded, continue with the remaining strings. Finally, return the result list.

You can also use a simpler compression method, add a newline character \0 at the end of each string, which also belongs to a string, so when decoding, just look for the newline character.

Code

  • import org.apache.commons.io.IOUtils;

import java.io.IOException;
import java.io.StringReader;
import java.util.LinkedList;
import java.util.List;

public class Encode_and_Decode_Strings {

    public class Codec {

        // Encodes a list of strings to a single string.
        public String encode(List<String> strs) {
            String res = "";
            for (String str : strs) res += str + '\0';
            return res;
        }

        // Decodes a single string to a list of strings.
        public List<String> decode(String s) throws IOException {
            return IOUtils.readLines(new StringReader(s));
        }
    }

    public class Codec_2nd_solution {

        // Encodes a list of strings to a single string.
        public String encode(List<String> strs) {
            StringBuffer sb = new StringBuffer();
            int size = strs.size();
            for (int i = 0; i < size; i++) {
                String str = strs.get(i);
                int length = str.length();
                sb.append(length + "/" + str);
            }
            return sb.toString();
        }

        // Decodes a single string to a list of strings.
        public List<String> decode(String s) {
            List<String> list = new ArrayList<String>();
            StringBuffer sb = new StringBuffer(s);

            int endIndex = 0;
            while (endIndex < s.length()) {
                int index = sb.toString().indexOf('/', endIndex); // @note: first `/` after index i
                int length = Integer.parseInt(sb.substring(endIndex, index));
                endIndex = index + length + 1;
                String str = sb.substring(index + 1, endIndex);
                list.add(str);
            }
            return list;
        }
    }

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

// Your Codec object will be instantiated and called as such:
// Codec codec = new 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));

  • // OJ: https://leetcode.com/problems/encode-and-decode-strings/
// Time: O(N)
// Space: O(1)
class Codec {
public:
    string encode(vector<string>& strs) {
        string ans;
        for (string &str : strs) {
            for (char c : str) {
                if (c == '$') ans.push_back(c);
                ans.push_back(c);
            }
            ans.push_back('$');
            ans.push_back('x');
        }
        return ans;
    }
    vector<string> decode(string s) {
        vector<string> ans;
        int i = 0, N = s.size();
        while (i < N) {
            string str;
            for (; i < N; ++i) {
                if (s[i] != '$') str.push_back(s[i]);
                else if (i + 1 < N && s[i + 1] == '$') str.push_back(s[i++]);
                else {
                    i += 2;
                    break;
                }
            }
            ans.push_back(str);
        }
        return ans;
    }
};

  • class Codec:
    def encode(self, strs):
        encoded = ""
        for s in strs:
            encoded += str(len(s)) + "#" + s
        return encoded

    def decode(self, s):
        decoded = []
        i = 0
        while i < len(s):
            delimiter = s.index("#", i)
            length = int(s[i:delimiter])
            decoded.append(s[delimiter+1 :delimiter+1+length])
            i = delimiter + 1 + length
        return decoded

############

'''
>>> s = 'abcde'
>>> '{:4}'.format(len(s)) + s
'   5abcde'
>>>
>>> '{:4}'.format(333)
' 333'
>>>
>>> '{:4}'.format(1234)
'1234'
>>> '{:4}'.format(12345)
'12345'

>>> '{0:4}'.format(12345)
'12345'
>>> '{1:4}'.format(12345)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: Replacement index 1 out of range for positional args tuple
>>> '{1:4}'.format(12345, 678910)
'678910'
>>> '{:4}'.format(12345, 678910)
'12345'
>>> '{0:4}'.format(12345, 678910)
'12345'
'''
class Codec: # no splitter
    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]) # so here potential overflow, if larger than int-max, need to clarify assumption with interviewer
            i += 4
            ans.append(s[i : i + size]) # note: exclusive for right index
            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));

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