Tutorial: Majority Element Interview Question and Solution in Python

In this tutorial, we will dive deep into the “Majority Element” interview question. We will explore what the problem entails, understand its significance, and work through various approaches to solve it efficiently using Python. The majority element is a crucial concept in algorithms and can often be encountered in coding interviews for technical positions.

Table of Contents

1. Introduction to the Majority Element Problem
2. Naive Approach
3. Hash Map Approach
4. Boyer-Moore Voting Algorithm
5. Sorting Approach
6. Divide and Conquer Approach
7. Comparative Analysis of Approaches
8. Python Implementation of Each Approach
9. Conclusion and Key Takeaways

1. Introduction to the Majority Element Problem

The Majority Element problem is a classic algorithmic problem that asks us to find an element in an array that appears more than n/2 times, where n is the size of the array. In simpler terms, we are looking for an element that occurs more frequently than any other element in the given array.

For example, consider the array: [3, 3, 4, 2, 4, 4, 2, 4, 4]. Here, the majority element is 4 since it appears 5 times, and the size of the array is 9.

This problem has practical applications in various fields, such as voting systems, data analysis, and signal processing, where finding an element that occurs more frequently can provide meaningful insights.

2. Naive Approach

One simple approach to solve this problem is by using a nested loop to count the occurrences of each element and check if it satisfies the majority condition. However, this approach is inefficient with a time complexity of O(n^2) since we compare each element with every other element.

3. Hash Map Approach

A more efficient approach involves utilizing a hash map to store the frequency of each element in the array. This way, we can traverse through the array once to populate the hash map and then find the element with the highest frequency. The time complexity of this approach is O(n), where n is the size of the array.

Here are the steps to solve the problem using the hash map approach:

1. Create an empty hash map to store element-frequency pairs.
2. Traverse through the array, and for each element:

• If the element is not in the hash map, add it with a frequency of 1.
• If the element is already in the hash map, increment its frequency.

1. Traverse through the hash map and find the element with the highest frequency.
2. Check if the frequency of the highest occurring element is greater than n/2, where n is the size of the array. If it is, return the element as the majority element.

4. Boyer-Moore Voting Algorithm

The Boyer-Moore Voting Algorithm is a clever and efficient approach that solves the majority element problem in O(n) time and O(1) space. It is based on the observation that the majority element must occur more frequently than all other elements combined. The algorithm involves maintaining a candidate element and a counter.

Here’s how the Boyer-Moore Voting Algorithm works:

1. Initialize candidate as None and count as 0.
2. Traverse through the array, and for each element:

• If count is 0, assign the current element to candidate and set count to 1.
• If the current element is the same as candidate, increment count.
• If the current element is different from candidate, decrement count.

1. After traversing through the array, the candidate will hold the majority element.
2. Verify if the candidate is the actual majority element by counting its occurrences in the array.

5. Sorting Approach

Another approach to solve the majority element problem involves sorting the array and then checking the element at index n/2, where n is the size of the array. Since the majority element occurs more than n/2 times, it will be the element at this index.

Here’s how the sorting approach works:

1. Sort the array in non-decreasing order.
2. Return the element at index n/2.

The sorting approach has a time complexity of O(n log n) due to the sorting step.

6. Divide and Conquer Approach

The divide and conquer approach is an elegant solution to the majority element problem that has a time complexity of O(n log n). It involves breaking down the array into smaller subproblems and then merging the solutions to these subproblems to find the overall majority element.

Here’s the high-level idea of the divide and conquer approach:

1. If the array has only one element, that element is the majority element.
2. Divide the array into two halves, left and right.
3. Recursively find the majority element in the left and right halves.
4. If the majority element is the same in both the left and right halves, it is the overall majority element.
5. If the majority elements in the left and right halves are different, count their occurrences in the entire array and return the one that satisfies the majority condition.

The divide and conquer approach can be a bit complex to implement but is an efficient alternative with better time complexity than the naive and sorting approaches.

7. Comparative Analysis of Approaches

Let’s compare the time complexities of the different approaches discussed:

• Naive Approach: O(n^2)
• Hash Map Approach: O(n)
• Boyer-Moore Voting Algorithm: O(n)
• Sorting Approach: O(n log n)
• Divide and Conquer Approach: O(n log n)

As we can see, the Boyer-Moore Voting Algorithm and the Hash Map Approach are the most efficient, both with a linear time complexity. The divide and conquer approach is also efficient, but it involves more complex implementation.

8. Python Implementation of Each Approach

Now, let’s implement each approach in Python for a better understanding. We’ll use a sample array [3, 3, 4, 2, 4, 4, 2, 4, 4] for demonstration purposes.

Hash Map Approach

def majority_element_hash_map(nums):
    frequency_map = {}
    n = len(nums)

    for num in nums:
        if num in frequency_map:
            frequency_map[num] += 1
        else:
            frequency_map[num] = 1

    for num, freq in frequency_map.items():
        if freq > n // 2:
            return num

    return None

nums = [3, 3, 4, 2, 4, 4, 2, 4, 4]
result = majority_element_hash_map(nums)
print("Majority Element:", result)

Boyer-Moore Voting Algorithm

def majority_element_boyer_moore(nums):
    candidate = None
    count = 0

    for num in nums:
        if count == 0:
            candidate = num
        if num == candidate:
            count += 1
        else:
            count -= 1

    return candidate

nums = [3, 3, 4,

 2, 4, 4, 2, 4, 4]
result = majority_element_boyer_moore(nums)
print("Majority Element:", result)

Sorting Approach

def majority_element_sorting(nums):
    nums.sort()
    n = len(nums)
    return nums[n // 2]

nums = [3, 3, 4, 2, 4, 4, 2, 4, 4]
result = majority_element_sorting(nums)
print("Majority Element:", result)

Divide and Conquer Approach

def majority_element_divide_conquer(nums, left, right):
    if left == right:
        return nums[left]

    mid = (left + right) // 2
    left_majority = majority_element_divide_conquer(nums, left, mid)
    right_majority = majority_element_divide_conquer(nums, mid + 1, right)

    if left_majority == right_majority:
        return left_majority

    left_count = sum(1 for num in nums[left:right+1] if num == left_majority)
    right_count = sum(1 for num in nums[left:right+1] if num == right_majority)

    return left_majority if left_count > right_count else right_majority

nums = [3, 3, 4, 2, 4, 4, 2, 4, 4]
result = majority_element_divide_conquer(nums, 0, len(nums) - 1)
print("Majority Element:", result)

9. Conclusion and Key Takeaways

In this tutorial, we explored the Majority Element interview question and different approaches to solving it efficiently using Python. We discussed the naive approach, hash map approach, Boyer-Moore Voting Algorithm, sorting approach, and divide and conquer approach.

Key takeaways from this tutorial:

• The Majority Element problem involves finding an element that appears more than n/2 times in an array.
• The hash map approach and Boyer-Moore Voting Algorithm are the most efficient solutions with linear time complexity.
• The divide and conquer approach has a slightly higher time complexity but is still efficient.
• The sorting approach is the least efficient among the discussed approaches due to its O(n log n) time complexity.

Remember to practice implementing these solutions on various inputs to build a strong grasp of the problem and its different solutions. This knowledge will not only help you ace interviews but also improve your problem-solving skills in general.