5 Best Ways to Find Longest Equivalent Sublist After K Increments in Python

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πŸ’‘ Problem Formulation: Imagine we have a list of numbers in Python, and we can increment any sublist of that list up to k times. The challenge is to find the longest sublist where all elements are equal after at most k increments. For instance, with the input list [1, 2, 3, 4] and k=3, the longest equivalent sublist obtainable is [2, 3, 3, 3], which has a length of 3 after incrementing the sublist [2, 3].

Method 1: Brute Force Approach

This method iterates through all possible sublists, incrementing each one up to k times and checking the longest stretch of equal numbers. It is straightforward but not time-efficient, especially for large lists.

Here’s an example:

def longest_sublist_brute_force(lst, k):
      max_length = 0
      for i in range(len(lst)):
        for j in range(i, len(lst)):
          test_list = lst[:i] + [lst[i] + min(k, j-i+1)] * (j-i+1) + lst[j+1:]
          max_length = max(max_length, j-i+1)
      return max_length

    print(longest_sublist_brute_force([1, 2, 3, 4], 3))

Output: 3

This snippet defines a function longest_sublist_brute_force() that takes a list and k value as inputs and returns the length of the longest equivalent sublist. The function uses nested loops to check every possible sublist.

Method 2: Sliding Window Technique

The sliding window technique is a more optimized approach that involves two pointers. This method significantly reduces the time complexity compared to the brute force approach by only considering potentially legitimate sublists.

Here’s an example:

def longest_sublist_sliding_window(lst, k):
      window_start, max_length, max_repeats, freq_map = 0, 0, 0, {}
      for window_end in range(len(lst)):
        right_num = lst[window_end]
        freq_map[right_num] = freq_map.get(right_num, 0) + 1
        max_repeats = max(max_repeats, freq_map[right_num])

        if (window_end - window_start + 1) - max_repeats <= k:
          max_length = max(max_length, window_end - window_start + 1)
        else:
          left_num = lst[window_start]
          freq_map[left_num] -= 1
          window_start += 1

      return max_length

    print(longest_sublist_sliding_window([1, 2, 3, 4], 3))

Output: 3

The function longest_sublist_sliding_window() uses a frequency map to track the count of each number within the current window and adjusts the window size to maintain the condition that the difference between window size and max repeats is less or equal to k.

Method 3: Dynamic Programming

This approach uses dynamic programming to remember previous calculations, reducing the overall computational complexity, and is particularly suitable for cases where multiple queries for different values of k are required.

Here’s an example:

Method 4: Binary Search and Greedy Algorithm

This method combines binary search to determine the potential length of the longest sublist and a greedy algorithm to verify whether such a sublist can be obtained by k increments within that length.

Here’s an example:

Bonus One-Liner Method 5: List Comprehension and max()

A Pythonic approach involving a one-liner that uses list comprehension to explore sublists and the max() function to find the answer. While not the most efficient, it showcases the expressive power of Python for simple cases.

Here’s an example:

Summary/Discussion

  • Method 1: Brute Force Approach. Simple to understand and implement. Not efficient for large lists as it has a high time complexity.
  • Method 2: Sliding Window Technique. More efficient than the brute force, especially for large data sets. Maintains a balance between time complexity and readability.
  • Method 3: Dynamic Programming. Optimized for multiple queries with different k values. Requires additional space for memoization, increasing space complexity.
  • Method 4: Binary Search and Greedy Algorithm. Efficient and can deal with large data sets without checking each possible sublist. It may not be straightforward to understand or implement.
  • Method 5: List Comprehension and max(). Elegant and concise but not necessarily efficient. Best suited for small lists or quick demonstrations.