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Encode and Decode Strings

Medium

Asked by:

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Topics:

Strings

Design an algorithm to encode a list of strings to a single string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Please implement the encode and decode methods.

Example 1:

Input: ["lint","code","love","you"]
Output: "4#lint4#code4#love3#you"
Explanation:
One possible encode method is: "4#lint4#code4#love3#you"

Example 2:

Input: ["we","say", ":","yes"]
Output: "2#we3#say1#:3#yes"
Explanation:
One possible encode method is: "2#we3#say1#:3#yes"

Constraints:

0 <= strs.length <= 200
0 <= strs[i].length <= 200
strs[i] contains any possible characters out of 256 valid ASCII characters.

Encode and Decode Strings

Medium

Asked by:

51 views

Topics:

Strings

Design an algorithm to encode a list of strings to a single string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Please implement the encode and decode methods.

Example 1:

Input: ["lint","code","love","you"]
Output: "4#lint4#code4#love3#you"
Explanation:
One possible encode method is: "4#lint4#code4#love3#you"

Example 2:

Input: ["we","say", ":","yes"]
Output: "2#we3#say1#:3#yes"
Explanation:
One possible encode method is: "2#we3#say1#:3#yes"

Constraints:

0 <= strs.length <= 200
0 <= strs[i].length <= 200
strs[i] contains any possible characters out of 256 valid ASCII characters.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What characters can the strings in the input list `strs` contain? Are there any restrictions on special characters or Unicode?
Can the input list `strs` be empty, or contain null or empty strings? What should the encode/decode functions return in these cases?
Is there a maximum length for the strings in the input list `strs`, or a maximum combined length for all strings? This could impact memory usage.
Is there any specific character or sequence that I should avoid using as a delimiter during encoding to prevent conflicts with the string content itself?
If there are multiple ways to encode and decode the strings, is there a preferred method or any specific constraints on the encoding scheme's efficiency?

Brute Force Solution

Approach

The encoding problem requires converting a list of strings into a single string, and the decoding problem requires doing the reverse. The brute force approach to encoding is straightforward, and for decoding, it involves trying every possible split point to see if it results in valid strings.

Here's how the algorithm would work step-by-step:

To encode, simply combine all the input strings into one big string.
Add some special markers or symbols to the combined string so you can tell where each original string starts and ends. This allows for easy separation during decoding.
To decode, start by examining the encoded string from the beginning.
Consider every possible position as a potential split point, based on the markers you placed during encoding.
For each possible split, check if the resulting 'strings' are valid according to how you encoded them.
If they are valid, keep those strings as part of a possible solution.
Repeat this process, exploring all possible split combinations, until you've processed the entire encoded string.
Finally, return the list of valid decoded strings that you've identified.

Code Implementation

def encode(list_of_strings):
    encoded_string = ""
    for string in list_of_strings:
        encoded_string += str(len(string)) + "#" + string
    return encoded_string

def decode(encoded_string):
    decoded_list = []
    string_index = 0
    # Need to iterate through encoded string
    while string_index < len(encoded_string):
        length_of_string = ""
        # Extract the length of the next string
        while encoded_string[string_index] != "#":
            length_of_string += encoded_string[string_index]
            string_index += 1

        string_index += 1
        length_of_string = int(length_of_string)
        # Extract the string using the determined length

        extracted_string = encoded_string[string_index : string_index + length_of_string]

        # Add extracted string to decoded list
        decoded_list.append(extracted_string)
        string_index += length_of_string

    return decoded_list

Try it yourself on LeetCode

Big(O) Analysis

Time Complexity

O(n^3) – Encoding takes O(n) time where n is the total length of all strings because we are concatenating all strings once. Decoding involves iterating through the encoded string of length 'n' (the combined length of all original strings). For each position, we try splitting the string which takes O(n) time and then validating if the resulting strings are valid which again could take O(n) time in worst case since we need to check all parts of the string, leading to O(n * n * n) or O(n^3) complexity in total.

Space Complexity

–

Optimal Solution

Approach

To encode a list of strings into one big string, we'll add a special marker to show where each string begins and ends. Decoding just reverses this process by using the markers to split the big string back into the original strings.

Here's how the algorithm would work step-by-step:

For encoding, go through each string in the list.
Before each string, put its length followed by a special character that's easy to spot and never used in the string itself. This acts as our marker.
Combine all these length-marker-string parts into one big encoded string.
For decoding, start reading the encoded string.
Find the first length marker.
Read the number representing the length, and then grab that many characters to form the first string.
Repeat this process, finding markers and extracting strings, until you've processed the whole encoded string.
You now have a list of all the original strings!

Code Implementation

def encode(list_of_strings):
    encoded_string = ""
    for string in list_of_strings:
        # Prepend length and delimiter for encoding.
        encoded_string += str(len(string)) + "#" + string

    return encoded_string

def decode(encoded_string):
    result = []
    index = 0

    while index < len(encoded_string):
        # Find the end of the length marker.
        delimiter_index = encoded_string.find("#", index)

        # Extract the length of the string.
        string_length = int(encoded_string[index:delimiter_index])

        # Extract the string based on the length.
        string = encoded_string[delimiter_index + 1: delimiter_index + 1 + string_length]
        result.append(string)
        index = delimiter_index + 1 + string_length

    return result

Try it yourself on LeetCode

Big(O) Analysis

Time Complexity

O(N) – Encoding iterates through each of the N strings in the input list. For each string, it calculates the length and prepends it with a delimiter, which takes constant time. The concatenation of these parts into the encoded string involves operations proportional to the total length of all strings, which is still within O(N) if we consider N to represent the total characters across all strings. Decoding iterates through the encoded string. For each string, it reads the length from the encoded string and extracts the substring of that length, which also takes O(1) operations per string effectively making the total time complexity O(N) where N is the total length of characters across all input strings.

Space Complexity

O(N) – During encoding, the algorithm builds a single large string by concatenating length markers and the input strings. The size of this encoded string can grow linearly with the total length of all input strings, which we can represent as N, where N is the total number of characters across all input strings. Similarly, during decoding, a list to hold the decoded strings is created, and in the worst case, the size of this list would be proportional to the total length of the encoded string (N). Therefore, the auxiliary space complexity is O(N).

Edge Cases

Case	How to Handle
Empty input list `strs`	The encode function should return an empty string, and the decode function should return an empty list if given an empty string.
List `strs` containing empty strings	The encode function should correctly encode empty strings and the decode function should be able to decode them, likely using a length prefix of 0.
List `strs` containing very long strings (near maximum string length allowed by the language)	Ensure the solution doesn't cause integer overflow when calculating length prefixes or during string concatenation and considers memory constraints.
Strings in `strs` containing the delimiter used in encoding	Choose a delimiter that's guaranteed not to appear in the input strings, or use an escape mechanism if a fixed delimiter is required.
Null or undefined input for `strs`	Throw an exception or return null/undefined, depending on the language and requirements, as a list is expected.
List `strs` containing unicode characters	Ensure the encoding and decoding process correctly handles multi-byte unicode characters without corrupting the data by considering UTF-8 encoding.
List `strs` with extremely large number of strings.	Consider memory implications when concatenating a very large number of strings, and explore using StringBuilder or similar to reduce string copy overhead.
Strings in `strs` starting with numeric characters.	The algorithm needs to correctly parse the length of the string when the string begins with a number.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What characters can the strings in the input list `strs` contain? Are there any restrictions on special characters or Unicode?
Can the input list `strs` be empty, or contain null or empty strings? What should the encode/decode functions return in these cases?
Is there a maximum length for the strings in the input list `strs`, or a maximum combined length for all strings? This could impact memory usage.
Is there any specific character or sequence that I should avoid using as a delimiter during encoding to prevent conflicts with the string content itself?
If there are multiple ways to encode and decode the strings, is there a preferred method or any specific constraints on the encoding scheme's efficiency?

Brute Force Solution

Approach

Here's how the algorithm would work step-by-step:

To encode, simply combine all the input strings into one big string.
Add some special markers or symbols to the combined string so you can tell where each original string starts and ends. This allows for easy separation during decoding.
To decode, start by examining the encoded string from the beginning.
Consider every possible position as a potential split point, based on the markers you placed during encoding.
For each possible split, check if the resulting 'strings' are valid according to how you encoded them.
If they are valid, keep those strings as part of a possible solution.
Repeat this process, exploring all possible split combinations, until you've processed the entire encoded string.
Finally, return the list of valid decoded strings that you've identified.

Code Implementation

def encode(list_of_strings):
    encoded_string = ""
    for string in list_of_strings:
        encoded_string += str(len(string)) + "#" + string
    return encoded_string

def decode(encoded_string):
    decoded_list = []
    string_index = 0
    # Need to iterate through encoded string
    while string_index < len(encoded_string):
        length_of_string = ""
        # Extract the length of the next string
        while encoded_string[string_index] != "#":
            length_of_string += encoded_string[string_index]
            string_index += 1

        string_index += 1
        length_of_string = int(length_of_string)
        # Extract the string using the determined length

        extracted_string = encoded_string[string_index : string_index + length_of_string]

        # Add extracted string to decoded list
        decoded_list.append(extracted_string)
        string_index += length_of_string

    return decoded_list

Try it yourself on LeetCode

Big(O) Analysis

Time Complexity

Space Complexity

–

Optimal Solution

Approach

Here's how the algorithm would work step-by-step:

For encoding, go through each string in the list.
Before each string, put its length followed by a special character that's easy to spot and never used in the string itself. This acts as our marker.
Combine all these length-marker-string parts into one big encoded string.
For decoding, start reading the encoded string.
Find the first length marker.
Read the number representing the length, and then grab that many characters to form the first string.
Repeat this process, finding markers and extracting strings, until you've processed the whole encoded string.
You now have a list of all the original strings!

Code Implementation

def encode(list_of_strings):
    encoded_string = ""
    for string in list_of_strings:
        # Prepend length and delimiter for encoding.
        encoded_string += str(len(string)) + "#" + string

    return encoded_string

def decode(encoded_string):
    result = []
    index = 0

    while index < len(encoded_string):
        # Find the end of the length marker.
        delimiter_index = encoded_string.find("#", index)

        # Extract the length of the string.
        string_length = int(encoded_string[index:delimiter_index])

        # Extract the string based on the length.
        string = encoded_string[delimiter_index + 1: delimiter_index + 1 + string_length]
        result.append(string)
        index = delimiter_index + 1 + string_length

    return result

Try it yourself on LeetCode

Big(O) Analysis

Time Complexity

Space Complexity

Edge Cases

Case	How to Handle
Empty input list `strs`	The encode function should return an empty string, and the decode function should return an empty list if given an empty string.
List `strs` containing empty strings	The encode function should correctly encode empty strings and the decode function should be able to decode them, likely using a length prefix of 0.
List `strs` containing very long strings (near maximum string length allowed by the language)	Ensure the solution doesn't cause integer overflow when calculating length prefixes or during string concatenation and considers memory constraints.
Strings in `strs` containing the delimiter used in encoding	Choose a delimiter that's guaranteed not to appear in the input strings, or use an escape mechanism if a fixed delimiter is required.
Null or undefined input for `strs`	Throw an exception or return null/undefined, depending on the language and requirements, as a list is expected.
List `strs` containing unicode characters	Ensure the encoding and decoding process correctly handles multi-byte unicode characters without corrupting the data by considering UTF-8 encoding.
List `strs` with extremely large number of strings.	Consider memory implications when concatenating a very large number of strings, and explore using StringBuilder or similar to reduce string copy overhead.
Strings in `strs` starting with numeric characters.	The algorithm needs to correctly parse the length of the string when the string begins with a number.