Solving Encode and Decode Strings in C#
1. The Problem: Encode and Decode Strings
The “Encode and Decode Strings” problem asks us to design an algorithm to convert a list of strings into a single string and then convert that single string back to the original list of strings.
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.
2. The Intuition: Separators vs. Length-Prefix
The core challenge is how to separate strings when they are joined together, especially if the strings themselves contain special characters.
- Unique Separator: Use a specific delimiter (e.g.,
<#string#>) to mark the end of each string. This is simple but can fail if the separator itself appears in the input strings. - Length-Prefix (Neetcode approach): Prepend each string with its length and a special character (e.g.,
4#word). This is much more robust because it tells the decoder exactly how many characters to read next, regardless of what’s inside the string.
3. Implementation (Version 1: Separator)
Here is the implementation using a private _separator field and standard C# string operations.
using System.Text;
public class Solution {
private string _separator = "<#string#>";
public string Encode(IList<string> strs) {
var sb = new StringBuilder();
foreach (string current in strs) {
sb.Append(current);
sb.Append(_separator);
}
return sb.ToString();
}
public List<string> Decode(string s) {
var arr = s.Split(_separator).ToList();
if (arr.Count > 0) {
arr.RemoveAt(arr.Count - 1);
}
return arr;
}
}
4. Implementation (Version 2: Length-Prefix / Neetcode)
This version follows the Neetcode approach, which uses the Length-Prefix strategy. This is more robust as it doesn’t rely on a unique separator that might appear in the data.
public class Solution {
public string Encode(IList<string> strs) {
string res = "";
foreach (string s in strs) {
res += s.Length + "#" + s;
}
return res;
}
public List<string> Decode(string s) {
List<string> res = new List<string>();
int i = 0;
while (i < s.Length) {
int j = i;
while (s[j] != '#') {
j++;
}
int length = int.Parse(s.Substring(i, j - i));
i = j + 1;
j = i + length;
res.Add(s.Substring(i, length));
i = j;
}
return res;
}
}
5. Step-by-Step Breakdown (Length-Prefix)
Step 1: The Encoding Logic
For every string, we calculate its length and append it to the result followed by a delimiter (like #) and then the string itself. For example, ["lint","code"] becomes "4#lint4#code".
Step 2: Finding the Length in Decoding
During decoding, we look for the first # to find where the length indicator ends. Everything before that # is parsed into an integer representing the string’s length.
Step 3: Reading the String
Once we have the length $L$, we know exactly that the next $L$ characters belong to the original string. We extract this substring and move our pointer forward to the start of the next length indicator.
6. Complexity Analysis
| Metric | Complexity | Why? |
|---|---|---|
| Time Complexity | O(N) | We process each character in the input once during encoding and once during decoding (where N is the total number of characters across all strings). |
| Space Complexity | O(N) | We store the encoded string and the resulting list, both of which are proportional to the total number of characters. |
7. Why use these approaches?
- Separator Version: Extremely simple to implement using built-in .NET methods. Best when you have control over the input data or can guarantee a unique separator.
- Length-Prefix (Neetcode): Much more robust and widely used in actual network protocols (like HTTP’s
Content-Length). It handles any characters in the input strings without escaping.
8. Summary
The Encode and Decode Strings problem is a classic exercise in data serialization. By choosing a clear delimiter and using efficient string building techniques, we can transform complex data structures into a format suitable for transmission and reconstruct them perfectly.
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