271_encode_and_decode_strings.md

Encode and Decode Strings

Problem Number: 271 Difficulty: Medium Category: String, Design LeetCode Link: https://leetcode.com/problems/encode-and-decode-strings/

Problem 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?

My Approach

I used a Delimiter-based approach to encode and decode strings. The key insight is to use a special delimiter that doesn't appear in the input strings to separate them.

Algorithm:

1. Encode: Join all strings with a special delimiter
2. Decode: Split the encoded string by the delimiter
3. Handle edge cases (empty array, empty strings)
4. Use a complex delimiter to avoid conflicts

Solution

The solution uses delimiter-based encoding and decoding. See the implementation in the solution file.

Key Points:

• Uses special delimiter to separate strings
• Handles empty arrays and empty strings
• Simple join/split approach
• Complex delimiter to avoid conflicts

Time & Space Complexity

Encode Time: O(n)

• Join operation: O(n) where n is total length of all strings

Decode Time: O(n)

• Split operation: O(n) where n is length of encoded string

Space Complexity: O(n)

• Encoded string: O(n)
• Decoded array: O(n)

Key Insights

1. Delimiter Choice: Using a complex delimiter avoids conflicts with input strings.

2. Join/Split: Simple join and split operations are efficient.

3. Edge Cases: Handle empty arrays and empty strings properly.

4. Delimiter Safety: Complex delimiter reduces chance of appearing in input.

5. Bidirectional: Encode and decode must be inverse operations.

6. Character Set: Works with any ASCII characters.

Mistakes Made

1. Simple Delimiter: Initially might use simple delimiter that conflicts with input.

2. Edge Cases: Not handling empty arrays or empty strings properly.

3. Complex Logic: Overcomplicating the encoding/decoding process.

4. Delimiter Conflicts: Using delimiter that might appear in input strings.

Related Problems

• Serialize and Deserialize Binary Tree (Problem 297): Serialize tree structure
• Serialize and Deserialize BST (Problem 449): Serialize BST
• Design HashMap (Problem 706): Design data structure
• Design HashSet (Problem 705): Design data structure

Alternative Approaches

1. Length Prefix: Use length prefix for each string - O(n) time
2. Escape Characters: Use escape characters for special cases - O(n) time
3. Base64 Encoding: Use base64 encoding - O(n) time

Common Pitfalls

1. Simple Delimiter: Using simple delimiter that conflicts with input.
2. Edge Cases: Not handling empty arrays or empty strings.
3. Complex Logic: Overcomplicating the encoding/decoding process.
4. Delimiter Conflicts: Using delimiter that might appear in input strings.
5. Character Set: Not considering all possible ASCII characters.

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Note: This is a design problem that demonstrates efficient string encoding and decoding with delimiters.