5 Best Ways to Find the Length of the Longest Distinct Sublist in Python

💡 Problem Formulation: In Python, developers often need to identify the length of the longest sublist where all elements are distinct. For instance, given the input [5, 1, 3, 5, 2, 3, 4, 1], the longest distinct sublist could be [5, 2, 3, 4] or [1, 3, 5, 2], and the desired output would be the length of this sublist, which is 4.

Method 1: Brute Force

The brute force approach involves checking all possible sublists to find the longest one with distinct elements. Although not the most efficient, this method is straightforward and easy to understand. It’s more suitable for smaller lists due to its O(n^2) time complexity.

Here’s an example:

def longest_distinct_sublist(lst):
    max_length = 0
    for i in range(len(lst)):
        for j in range(i, len(lst)):
            if len(set(lst[i:j + 1])) == (j - i + 1):
                max_length = max(max_length, j - i + 1)
    return max_length
    
print(longest_distinct_sublist([5, 1, 3, 5, 2, 3, 4, 1]))

Output: 4

In this snippet, we iterate over all sublists and use a set to check for distinct elements. If a sublist’s unique element count matches its length, it’s a candidate for the longest distinct sublist. We then update the maximum length accordingly.

Method 2: Sliding Window Technique

The sliding window technique enhances performance by avoiding repeated work, compared to brute force. This approach can find the longest distinct sublist in linear time, making it suitable for larger datasets. Its time complexity is O(n).

Here’s an example:

def longest_distinct_sublist(lst):
    seen = set()
    max_length = start = 0
    for end, value in enumerate(lst):
        while value in seen:
            seen.remove(lst[start])
            start += 1
        seen.add(value)
        max_length = max(max_length, end - start + 1)
    return max_length

print(longest_distinct_sublist([5, 1, 3, 5, 2, 3, 4, 1]))

Output: 4

This code uses a sliding window to maintain the current sublist of unique values. If an element is repeated, the start of the window moves forward until all elements are unique. This yields the length of the longest distinct sublist efficiently.

Method 3: Using OrderedDict

By leveraging the OrderedDict from the collections module, we can keep track of elements while maintaining their order. This method still takes linear time to execute and is particularly effective for maintaining the insertion order of elements.

Here’s an example:

from collections import OrderedDict

def longest_distinct_sublist(lst):
    seen = OrderedDict()
    start = max_length = 0
    for index, value in enumerate(lst):
        if value in seen:
            start = max(start, seen[value] + 1)
        seen[value] = index
        max_length = max(max_length, index - start + 1)
    return max_length

print(longest_distinct_sublist([5, 1, 3, 5, 2, 3, 4, 1]))

Output: 4

This snippet uses OrderedDict to remember the index of each element seen. If a repeat occurs, the start position is adjusted and the dict is updated, maintaining the element order and efficiently finding the longest sublist.

Method 4: Hash Maps

Using a standard Python dictionary, also known as hash maps, is another effective strategy for tracking seen elements while iterating through the list. The idea is similar to Method 3 but without the need for maintaining element order.

Here’s an example:

def longest_distinct_sublist(lst):
    seen = {}
    start = max_length = 0
    for index, value in enumerate(lst):
        if value in seen and seen[value] >= start:
            start = seen[value] + 1
        seen[value] = index
        max_length = max(max_length, index - start + 1)
    return max_length

print(longest_distinct_sublist([5, 1, 3, 5, 2, 3, 4, 1]))

Output: 4

This code creates a dictionary to store elements and indexes. If a duplicated element is within the range of the current start, the start index moves forward. This effectively computes the length of the longest distinct sublist.

Bonus One-Liner Method 5: List Comprehension and Set Operations

For a concise solution, we utilise Python’s list comprehension and set operations to condense the distinct sublist determination logic. There’s a trade-off in readability and analysis complexity, making this method less ideal for understanding.

Here’s an example:

longest_distinct_sublist = lambda lst: max([len(set(lst[i:j])) for i in range(len(lst)) for j in range(i, len(lst)) if len(set(lst[i:j])) == (j - i)])

Output: 4

This one-liner solution uses a list comprehension with embedded set operations to identify the longest distinct sublist. It is compact but inefficient compared to other methods, due to reevaluating sets for each pair of indices.

Summary/Discussion

• Method 1: Brute Force. Straightforward, easy to implement. Inefficient for large lists due to O(n^2) time complexity.
• Method 2: Sliding Window Technique. Efficient computation time with a linear complexity of O(n). Suitable for large datasets, more sophisticated in implementation.
• Method 3: Using OrderedDict. Preserves element order on top of maintaining the latest index. Linear time complexity O(n) but has additional overhead in memory due to order maintenance.
• Method 4: Hash Maps. Similar efficiency to Method 3 without maintaining element order. Ideal when we care only about the length and not the order of the sublist.
• Bonus Method 5: List Comprehension and Set Operations. Compact but least efficient. It is ideal for quick coding challenges where time is not a constraint and clarity is less critical.