Encode and Decode Strings

mediumFREEArrays & HashingBlind 75

Design an algorithm to encode a list of strings into a single string and decode it back.

Implement encode(strs), which takes a list of strings and returns one string, and decode(s), which takes that string and returns the original list. For every valid input, decode(encode(strs)) must equal strs exactly (same order, same count, empty strings preserved).

Input: a list of strings (the example shows it comma-separated for display only). Output: the decoded strings, one per line.

Example

Input

hello,world

Output

hello
world

Reasoning

Input: a list of $n = 2$ strings, hello and world (shown comma-separated only for display), each 5 characters long. encode must turn the whole list into one string; decode must turn that string back into exactly the original list.
The requirement: decoding must undo encoding, $\text{decode}(\text{encode}(L)) = L$ for every list $L$ , including the empty list, empty strings, and strings that contain commas or any other character.
Why plain joining fails: the lists [hello, world], [hellow, orld] and [helloworld] all glue into the same 10 characters. If two different lists produce the same encoded string, no decoder can tell which one it came from, so the encoding must record where each string ends without relying on any character being absent from the data.
Output: the recovered list has 2 entries, printed one per line: hello then world.
Notice that a comma delimiter would break on a list holding the string "a,b" and an empty string: it must come back as 2 strings, not 3.

Constraints

0 <= len(strs) <= 200
0 <= len(strs[i]) <= 200
strs[i] may contain any character

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Concept primer

Background Knowledge

The "Encode and Decode Strings" problem involves designing an algorithm to encode a list of strings into a single string and then decode it back. This problem falls under the topic of Hash Maps & Counting, which are fundamental data structures and techniques in computer science. A hash map (also known as a hash table) is a data structure that stores key-value pairs in an array using a hash function to map keys to indices of the array. In the context of this problem, we can use a hash map to keep track of the length of each string.

The concept of encoding and decoding is also crucial in this problem. Encoding refers to the process of converting data into a coded form, while decoding is the reverse process of converting the coded form back into the original data. In this case, we need to encode a list of strings into a single string and then decode it back into the original list of strings. This requires careful consideration of how to represent the strings in the encoded form, taking into account the variable lengths of the strings.

To solve this problem, we need to understand how to manipulate strings and use data structures such as hash maps to keep track of the lengths of the strings. We also need to consider how to handle the encoding and decoding process, including how to separate the strings in the encoded form and how to reconstruct the original strings during decoding. This involves using techniques such as string concatenation and substring extraction.

Algorithm/Approach

The general approach to solving this problem involves using a combination of string manipulation techniques and data structures such as hash maps. One possible approach is to use a length-encoded scheme, where the length of each string is encoded as a prefix to the string itself. This allows us to easily separate the strings during decoding. We can use a hash map to keep track of the lengths of the strings, or we can use a simpler approach that relies on the properties of the input strings.

Step-by-Step Strategy

To implement the solution, we can follow these steps:

Determine the encoding scheme: Decide how to represent the strings in the encoded form, including how to handle the variable lengths of the strings.
Encode the strings: Iterate through the input strings and apply the encoding scheme to each string, concatenating the encoded strings into a single string.
Decode the string: Iterate through the encoded string and apply the decoding scheme to extract the original strings, using the encoded length information to separate the strings.
Handle edge cases: Consider how to handle edge cases, such as empty strings or strings with special characters.

Common Pitfalls

When implementing the solution, we need to watch out for the following common pitfalls:

Incorrectly handling the encoding and decoding of string lengths
Failing to account for edge cases, such as empty strings or strings with special characters
Using an inefficient encoding scheme that leads to unnecessary complexity or performance issues

Time & Space Complexity

The expected time complexity of the solution is O(n), where n is the total length of the input strings, since we need to iterate through each string to encode and decode it. The space complexity is also O(n), since we need to store the encoded string and the decoded strings. However, the exact time and space complexity will depend on the specific encoding scheme and implementation details.

solution.py

Python 3.13

Test results