Majority Element Moore's Voting Algorithm in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Moore's Voting Algorithm on a given list

What is the output of the following Python code that implements Moore's Voting Algorithm to find the majority element?

DSA Python

def majority_element(nums):
    count = 0
    candidate = None
    for num in nums:
        if count == 0:
            candidate = num
        count += (1 if num == candidate else -1)
    return candidate

print(majority_element([2, 2, 1, 1, 1, 2, 2]))

A Error

B 1

C None

D 2

💡 Hint

The majority element is the one that appears more than half the time.

The algorithm picks candidate 2 and confirms it as majority because it appears 4 times out of 7.

🧠 Conceptual

intermediate

Understanding the count variable in Moore's Voting Algorithm

In Moore's Voting Algorithm, what does the variable 'count' represent during the iteration over the list?

A The number of times the current candidate has been seen minus the number of times other elements have been seen

B The total number of elements processed so far

C The index of the current candidate in the list

D The number of unique elements encountered

💡 Hint

Think about how the algorithm balances the candidate count.

Count increases when the current element matches the candidate and decreases otherwise, tracking net support.

❓ Predict Output

advanced

Output when no majority element exists

What will the following code output when run on a list with no majority element?

DSA Python

def majority_element(nums):
    count = 0
    candidate = None
    for num in nums:
        if count == 0:
            candidate = num
        count += (1 if num == candidate else -1)
    return candidate

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

A 2

B 1

C 3

D 4

💡 Hint

Trace the algorithm step by step to see the final candidate.

The candidate ends as 3 after the last iteration, but it is not a true majority.

🔧 Debug

advanced

Identify the error in this Moore's Voting Algorithm implementation

What error will this code produce when run?

DSA Python

def majority_element(nums):
    count = 0
    candidate = None
    for num in nums:
        if count = 0:
            candidate = num
        count += (1 if num == candidate else -1)
    return candidate

print(majority_element([3,3,4,2,4,4,2,4,4]))

A SyntaxError

B TypeError

C NameError

D No error, outputs 4

💡 Hint

Check the if condition syntax carefully.

The code uses '=' instead of '==' in the if condition, causing a SyntaxError.

🧠 Conceptual

expert

Why Moore's Voting Algorithm requires a second pass in some cases

Why might Moore's Voting Algorithm require a second pass over the list after finding a candidate?

A To sort the list for easier majority detection

B To verify that the candidate actually appears more than half the time

C To count the total number of unique elements

D To reset the candidate and count variables

💡 Hint

The algorithm assumes a majority element exists but does not confirm it.

The first pass finds a candidate, but the second pass confirms if it truly is the majority.