Majority Element Moore's Voting Algorithm in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Majority Element in an array?

The Majority Element is the element that appears more than half the size of the array (n/2 times).

intermediate

What is the main idea behind Moore's Voting Algorithm?

Moore's Voting Algorithm finds the majority element by maintaining a candidate and a count. It increases the count when the same element appears and decreases it when a different element appears, resetting the candidate when count reaches zero.

intermediate

Why does Moore's Voting Algorithm work for finding the majority element?

Because the majority element appears more than half the time, it will remain as the candidate after canceling out all other elements.

beginner

What is the time complexity of Moore's Voting Algorithm?

The time complexity is O(n) because it scans the array twice: once to find the candidate and once to verify it.

intermediate

What happens if there is no majority element in the array when using Moore's Voting Algorithm?

The algorithm will still return a candidate, but a second pass is needed to verify if it actually appears more than n/2 times. If not, there is no majority element.

What does Moore's Voting Algorithm maintain while scanning the array?

A A sorted list of elements

B A candidate element and a count

C A hash map of element frequencies

D A stack of elements

Moore's Voting Algorithm keeps track of a candidate and a count to find the majority element efficiently.

What is the minimum number of times the majority element appears in the array?

A At least once

B Exactly n/2 times

C Less than n/2 times

D More than n/2 times

By definition, the majority element appears more than half the size of the array.

What should you do after finding a candidate with Moore's Voting Algorithm?

A Sort the array

B Return it immediately

C Verify if it appears more than n/2 times

D Count all elements again

Verification is needed to confirm the candidate is truly the majority element.

What is the space complexity of Moore's Voting Algorithm?

A O(1)

B O(n)

C O(log n)

D O(n^2)

The algorithm uses only a few variables, so space complexity is constant.

If the count reaches zero during the scan, what does Moore's Voting Algorithm do?

A Changes the candidate to the current element

B Stops the algorithm

C Increases the count

D Removes the current element

When count is zero, the algorithm picks the current element as the new candidate.

Explain step-by-step how Moore's Voting Algorithm finds the majority element in an array.

Think about how the algorithm cancels out different elements.

Describe why Moore's Voting Algorithm requires a second pass to verify the candidate.

The algorithm assumes a majority element exists but needs proof.