5 Best Ways to Find Length of Longest Sublist with Sum Zero in Python

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πŸ’‘ Problem Formulation: In various scenarios within data processing and algorithm design, it becomes necessary to identify sublists within a list of integers where the sum of the elements is zero. The specific challenge addressed in this article is to find the length of the longest such sublist. Given an input list such as [1, 2, -3, 3, 1, -1, 2] the desired output would be 4, corresponding to the longest sublist [2, -3, 3, 1] or [3, 1, -1, 2] where the sum is zero.

Method 1: Using HashMap

This method involves creating a hashmap that stores the sum of elements at each index as keys and their respective indices as values. We iteratively compute the cumulative sum and check whether the sum has been seen before or is zero. If so, we update the length of the longest sublist. This method is efficient with O(n) time complexity.

Here’s an example:

def maxSubArrayLen(nums):
    sum_map = {0: -1}
    max_len = 0
    cum_sum = 0
    for i, num in enumerate(nums):
        cum_sum += num
        if cum_sum not in sum_map:
            sum_map[cum_sum] = i
        else:
            max_len = max(max_len, i - sum_map[cum_sum])
    return max_len

print(maxSubArrayLen([1, 2, -3, 3, 1, -1, 2]))

Output:

4

This code snippet defines a function maxSubArrayLen() that takes a list as input and returns the length of the longest sublist with sum zero. It uses a hashmap to track the cumulative sum at each index and identifies the longest zero-sum sublist by looking at previous occurrences of the same sum.

Method 2: Brute Force

The brute force approach checks every possible sublist and calculates its sum, updating the length of the longest sublist if a zero sum is found. This method is very straightforward but inefficient, with O(n^2) time complexity, making it unsuitable for large lists.

Here’s an example:

def findMaxLength(nums):
    max_len = 0
    for start in range(len(nums)):
        current_sum = 0
        for end in range(start, len(nums)):
            current_sum += nums[end]
            if current_sum == 0:
                max_len = max(max_len, end - start + 1)
    return max_len

print(findMaxLength([1, 2, -3, 3, 1, -1, 2]))

Output:

4

The function findMaxLength() iterates over all possible sublists, computes the sum for each, and updates the maximum length of the sublist that sums to zero. It’s simple but has a higher time complexity compared to the hashmap method.

Method 3: Prefix Sum and Lookup

Similar to the hashmap approach, we employ a prefix sum concept to keep a cumulative sum as we traverse the list. Alongside, we maintain a lookup to store the first occurrence index of a cumulative sum. This helps to quickly identify when a zero sum sublist is repeated and calculate its length.

Here’s an example:

def longestSublistZeroSum(nums):
    prefix_sum = 0
    sum_lookup = {0: -1}
    max_len = 0
    for idx, num in enumerate(nums):
        prefix_sum += num
        if prefix_sum not in sum_lookup:
            sum_lookup[prefix_sum] = idx
        else:
            max_len = max(max_len, idx - sum_lookup[prefix_sum])
    return max_len

print(longestSublistZeroSum([1, 2, -3, 3, 1, -1, 2]))

Output:

4

The function longestSublistZeroSum() uses a prefix sum and a lookup dictionary to find the longest sublist with a sum of zero. If the prefix sum has been seen before, we can instantly determine the length of a zero-sum sublist, making this method highly efficient.

Method 4: Using itertools

This method leverages Python’s itertools library to generate all possible sublists using combinations, followed by checking their sum. While still having a high complexity, this method can be succinct and leverages efficient iteration techniques from itertools.

Here’s an example:

import itertools

def longestZeroSumSublist(nums):
    max_len = 0
    for i in range(len(nums) + 1):
        for combination in itertools.combinations(nums, i):
            if sum(combination) == 0:
                max_len = max(max_len, i)
    return max_len

print(longestZeroSumSublist([1, 2, -3, 3, 1, -1, 2]))

Output:

4

The function longestZeroSumSublist() uses itertools to generate all possible sublist combinations, checks if their sum equals zero, and updates the maximum length accordingly. It can process small to medium-sized lists but becomes computationally expensive as list size grows.

Bonus One-Liner Method 5: List Comprehension and Max Function

This concise one-liner makes use of Python’s list comprehension combined with the max function to iterate through all sublists, calculate their sum, and return the length of the longest zero-sum sublist. It is elegant and leverages Python’s expressive syntax and built-in functions.

Here’s an example:

print(max((j-i for i in range(len(nums)) for j in range(i+1, len(nums)+1) if sum(nums[i:j]) == 0), default=0))

Output:

4

This one-liner computes the length of the longest sublist with a sum of zero by nesting loops within a generator expression and passing it to the max function. This solution demonstrates the power of Python’s compact syntax, though it comes with the tradeoff of reduced readability and higher time complexity.

Summary/Discussion

  • Method 1: HashMap. Efficient use of a hashmap to track cumulative sums. Very fast with O(n) time complexity. Might not be the most intuitive for beginners.
  • Method 2: Brute Force. Simple and straightforward to understand. However, it’s inefficient, with O(n^2) time complexity, making it unsuitable for large datasets.
  • Method 3: Prefix Sum and Lookup. Similar efficiency to the hashmap with O(n) time complexity. Quite streamlined but requires understanding of prefix sums and lookup strategy.
  • Method 4: Using itertools. Leverages standard library itertools for combination generation. More concise but still suffers from high time complexity for large lists.
  • Bonus Method 5: Compact one-liner that showcases Python’s expressive power. Less readable and not efficient for large datasets but exemplary of Pythonic brevity.