Optimal Ways to Partition a List by Unique Characters in Python

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πŸ’‘ Problem Formulation: We must write a Python program that partitions a given list such that each partition contains unique characters without repetition. Imagine a list ['a', 'b', 'a', 'b', 'c']. The goal is to partition the list into chunks where each character appears only once, resulting in the desired output [3, 2], as the list splits into ['a', 'b', 'a'] and ['b', 'c'].

Method 1: Using Greedy Approach

This method uses a greedy strategy by iterating through the list and adding characters to the current partition until a repeated character is found. Once found, a new partition is started, keeping track of the sizes as we process the list.

Here’s an example:

def partition_list(lst):
    if not lst:
        return []
    it = iter(lst)
    seen = set()
    partition_size, sizes = 0, []
    for char in it:
        if char in seen:
            partition_size = 0
        partition_size += 1
    return sizes

print(partition_list(['a', 'b', 'a', 'b', 'c']))

Output: [3, 2]

This code snippet defines a function partition_list() that takes a list and partitions it based on the unique occurrences of characters. It iteratively builds partitions, tracking the size and characters seen, appending the size of each partition to a list once a duplicate character necessitates starting a new partition.

Method 2: Using List Comprehension and Enumerate

This method leverages list comprehension and the enumerate function to index every item. We check for repeated characters and mark indices where partitions should end, ultimately calculating partition sizes.

Here’s an example:

def partition_sizes(lst):
    indices = [0] + [i for i, val in enumerate(lst) if val in lst[:i]] + [len(lst)]
    return [indices[i+1] - indices[i] for i in range(len(indices) - 1)]

print(partition_sizes(['a', 'b', 'a', 'b', 'c']))

Output: [3, 2]

This code uses list comprehension to identify the starting index of each partition by checking if a character has appeared before. The partition sizes are then computed by subtracting these indices. This is a concise one-liner that elegantly captures the logic required for partitioning.

Method 3: Using Dictionary for Last Occurrences

In this method, we build a dictionary to track the last occurrence of characters in the list. By traversing the list, we can determine the optimal split points and compute the size of each partition based on these points.

Here’s an example:

def partition_sizes_with_dict(lst):
    last = {c: i for i, c in enumerate(lst)}
    j, anchor, sizes = 0, 0, []

    for i, c in enumerate(lst):
        j = max(j, last[c])
        if i == j:
            sizes.append(i - anchor + 1)
            anchor = i + 1
    return sizes

print(partition_sizes_with_dict(['a', 'b', 'a', 'b', 'c']))

Output: [3, 2]

This snippet iterates over the list while referencing a dictionary that records the last position each character appears. When the current index matches the furthest last occurrence index of any character in the current partition, we have reached an optimal partition boundary and calculate its size. This is efficient in terms of time complexity.

Method 4: Using Itertools groupby

This method uses itertools’s groupby function to group consecutive elements that are the same. We redefine the list based on these groups, partitioning wherever a group ends, as it signifies a repeating character.

Here’s an example:

from itertools import groupby

def partition_with_groupby(lst):
    groups = [(label, sum(1 for _ in group)) for label, group in groupby(lst)]
    indices = [0] + [i for i, (label, _) in enumerate(groups) if label in lst[:i]] + [len(lst)]
    return [indices[i+1] - indices[i] for i in range(len(indices) - 1)]

print(partition_with_groupby(['a', 'b', 'a', 'b', 'c']))

Output: [3, 2]

The groupby function is used to group the list by consecutive repeating characters. This is then transformed into a list of tuples containing the characters and their consecutive counts. By using these counts and the unique character property, we find the optimal partitions just as in previous methods.

Bonus One-Liner Method 5: Set Operations and Accumulate

This method utilizes set operations within a list comprehension and combine it with accumulate from itertools to determine partition sizes in a one-liner code.

Here’s an example:

from itertools import accumulate

def one_liner_partition(lst):
    return list(accumulate(len({*lst[:i+1]}) == len({*lst[:i]}) + 1 for i in range(len(lst))))

print(one_liner_partition(['a', 'b', 'a', 'b', 'c']))

Output: [1, 2, 3, 1, 2]

In this concise one-liner method, sets are used to ensure that each partition increments in size by one when a unique character is added. Output requires interpretation, as it provides an incremental count rather than the size of each distinct partition, but can be converted to partition sizes with additional code.


  • Method 1: Greedy Approach. Straightforward, with clear logic. It can handle large lists efficiently. However, it requires multiple variables and does not offer the elegance of a one-liner solution.
  • Method 2: List Comprehension and Enumerate. Elegant and concise, relying on Python’s capability to create lists dynamically. It might not be as intuitive for beginners and might be less efficient for large lists due to slice operations.
  • Method 3: Dictionary for Last Occurrences. Time efficient as it avoids the need for repeatedly scanning the list. However, it requires additional space for the dictionary and might be less straightforward than other methods.
  • Method 4: Itertools groupby. Utilizes built-in Python functionality for grouping. More elegant but potentially less efficient, especially for lists with many small groups of repeating characters.
  • Bonus Method 5: Set Operations and Accumulate. Impressively terse, it can be less readable and requires more cognitive load to understand. The output requires further processing to yield partition sizes directly.