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Split Message Based on Limit

Difficulty: Hard

Problem Description

You are given a string, message, and a positive integer, limit. You must split message into one or more parts based on limit. Each resulting part should have the suffix "<a/b>", where "b" is the total number of parts and "a" is the index of the part, starting from 1. Additionally, the length of each resulting part (including its suffix) should be equal to limit, except for the last part whose length can be at most limit. The resulting parts should be formed such that when their suffixes are removed and they are all concatenated in order, they should be equal to message. Also, the result should contain as few parts as possible. Return the parts message would be split into as an array of strings. If it is impossible to split message as required, return an empty array.

Key Insights

  • Each part, including its suffix, must have a length equal to limit, except possibly the last part.
  • The index and total number of parts must be included in the suffix.
  • We need to determine the number of parts required to fit the entire message within the given constraints.
  • The suffix length varies based on the total number of parts, which affects how much of the message can be included in each part.

Space and Time Complexity

Time Complexity: O(n) - where n is the length of the message, as we need to iterate through the message to construct the parts. Space Complexity: O(k) - where k is the number of parts created, as we store each part in an array.

Solution

To solve the problem, we approach it as follows:

  1. First, we determine the minimum number of parts required by considering the maximum length of the suffix that can be added to each part.
  2. We use a loop to construct each part of the message, ensuring that we respect the limit for each part's length.
  3. The index of each part and the total number of parts are calculated dynamically as we build each part.
  4. If at any point we cannot create a valid part, we return an empty array.

We can utilize a list to store the parts and concatenate the message accordingly.

Code Solutions

def split_message(message: str, limit: int) -> List[str]:
    # Calculate the minimum number of parts needed
    total_chars = len(message)
    suffix_length = len("<a/b>")  # Length of the suffix
    parts = []

    # Try to find the number of parts
    for num_parts in range(1, total_chars + 1):
        # Calculate the maximum characters per part, considering the suffix
        max_chars_per_part = limit - suffix_length
        # Calculate the total length needed to fit the message
        if num_parts * max_chars_per_part < total_chars:
            continue
        
        # Now we know we can split into num_parts parts
        index = 1
        current_position = 0
        while index <= num_parts:
            # Calculate the suffix for the current part
            part_suffix = f"<{index}/{num_parts}>"
            part_length = limit - len(part_suffix)
            # Get the current part from the message
            current_part = message[current_position:current_position + part_length]
            if index == num_parts:  # Last part, may be shorter
                current_part = message[current_position:]
            parts.append(current_part + part_suffix)
            current_position += part_length
            index += 1

        return parts

    return []
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