Majority Element Moore's Voting Algorithm in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the count variable to zero.

DSA Javascript

let count = [1];

Drag options to blanks, or click blank then click option'

A1

B-1

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing count to 1 or null instead of 0.

We start the count at zero because no elements have been counted yet.

2fill in blank

medium

Complete the code to assign the first element of the array as the initial candidate.

DSA Javascript

let candidate = nums[1];

Drag options to blanks, or click blank then click option'

A(0)

B.0

C[0]

D{0}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or curly braces instead of square brackets.

The first element of the array is accessed using square brackets with index 0.

3fill in blank

hard

Fix the error in the if condition to check if the current element equals the candidate.

DSA Javascript

if (nums[i] [1] candidate) {

Drag options to blanks, or click blank then click option'

A==

B===

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which assigns instead of compares.

Using '==' which is less strict.

Use the strict equality operator '===' in JavaScript to compare values and types.

4fill in blank

hard

Fill both blanks to update candidate and reset count when count reaches zero.

DSA Javascript

if (count === 0) {
  candidate = nums[1];
  count [2] 1;
}

Drag options to blanks, or click blank then click option'

A[i]

B= 1

C+= 1

D(i)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for array access.

Using '+=' instead of '=' to reset count.

When count is zero, assign the current element as candidate and reset count to 1.

5fill in blank

hard

Fill all three blanks to correctly update count based on element comparison.

DSA Javascript

count [1] (nums[i] === candidate) ? [2] : [3];

Drag options to blanks, or click blank then click option'

A+=

B1

C-1

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '+=' sets an absolute value (1 or -1) every time instead of accumulating changes, preventing count from reaching zero properly.

Use '+=' to increment count by 1 if the current element equals the candidate, or decrement by 1 otherwise: count += (nums[i] === candidate) ? 1 : -1;

Practice

1. What is the main purpose of Moore's Voting Algorithm in arrays?

easy

A. To find the element that appears more than half the time

B. To sort the array in ascending order

C. To find the smallest element in the array

D. To reverse the array elements

Solution

1. Step 1: Understand the problem Moore's Voting Algorithm solves

   It is designed to find the majority element, which is the element that appears more than half the times in the array.

2. Step 2: Compare with other options

   Sorting, finding smallest, or reversing are unrelated to majority element detection.

3. Final Answer:

   To find the element that appears more than half the time -> Option A

4. Quick Check:

   Majority element = element appearing > n/2 times [OK]

Hint: Majority element appears more than half the array length [OK]

Common Mistakes:

• Confusing majority element with most frequent element
• Thinking it sorts the array
• Assuming it finds smallest or largest element

2. Which of the following is the correct initialization step in Moore's Voting Algorithm in JavaScript?

easy

A. let count = 1, candidate = 0;

B. let count = 0, candidate = null;

C. let candidate = 0, count = null;

D. let count = null, candidate = 1;

Solution

1. Step 1: Identify correct variable initialization

   Moore's Voting Algorithm starts with count = 0 and candidate = null (or undefined) to indicate no candidate chosen yet.

2. Step 2: Check other options for correctness

   Options A, C, and D initialize count or candidate incorrectly, which can cause logic errors.

3. Final Answer:

   let count = 0, candidate = null; -> Option B

4. Quick Check:

   Initialize count=0 and candidate=null [OK]

Hint: Start count at 0 and candidate as null before loop [OK]

Common Mistakes:

• Starting count at 1 instead of 0
• Assigning candidate a number before selection
• Swapping count and candidate initialization

3. What will be the output of the following JavaScript code?

function majorityElement(nums) {
  let count = 0, candidate = null;
  for (const num of nums) {
    if (count === 0) candidate = num;
    count += (num === candidate) ? 1 : -1;
  }
  return candidate;
}
console.log(majorityElement([2,2,1,1,1,2,2]));

medium

A. Error

B. 1

C. null

D. 2

Solution

1. Step 1: Trace the loop and candidate updates

   Initial count=0, candidate=null. Iteration updates candidate and count as follows:
   - num=2: count=0, candidate=2, count=1
   - num=2: count=1, same candidate, count=2
   - num=1: count=2, different candidate, count=1
   - num=1: count=1, different candidate, count=0
   - num=1: count=0, candidate=1, count=1
   - num=2: count=1, different candidate, count=0
   - num=2: count=0, candidate=2, count=1

2. Step 2: Final candidate after loop

   The candidate is 2 after the loop ends, which is the majority element.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Majority element found = 2 [OK]

Hint: Track count changes; candidate updates when count hits zero [OK]

Common Mistakes:

• Assuming candidate never changes
• Confusing count increments and decrements
• Returning count instead of candidate

4. Identify the bug in this Moore's Voting Algorithm implementation:

function majorityElement(nums) {
  let count = 1, candidate = nums[0];
  for (let i = 1; i < nums.length; i++) {
    if (nums[i] === candidate) {
      count++;
    } else {
      count--;
      if (count === 0) {
        candidate = nums[i];
        count = 1;
      }
    }
  }
  return candidate;
}

medium

A. Initialization of count should be 0, not 1

B. Candidate should be initialized to null, not nums[0]

C. No bug, code is correct

D. Loop should start from i = 0, not i = 1

Solution

1. Step 1: Analyze initialization and loop

   Starting count at 1 and candidate as nums[0] is a valid approach. Loop starts at i=1 to compare remaining elements.

2. Step 2: Check logic inside loop

   Count increments if current equals candidate, else decrements. When count hits zero, candidate updates and count resets to 1. This matches Moore's Voting Algorithm logic.

3. Final Answer:

   No bug, code is correct -> Option C

4. Quick Check:

   Initialization and loop logic correct [OK]

Hint: Starting count=1 with first element candidate is valid [OK]

Common Mistakes:

• Thinking count must start at 0
• Assuming candidate must be null initially
• Starting loop at wrong index

5. Given an array where no element appears more than half the time, what will Moore's Voting Algorithm return?
Example: [1, 2, 3, 4]

hard

A. An incorrect majority element

B. null or undefined (no majority element)

C. The first element of the array

D. The element with the highest frequency

Solution

1. Step 1: Understand Moore's Voting Algorithm assumption

   The algorithm assumes a majority element exists (appears more than half the time). If none exists, it still returns a candidate but it may not be a true majority.

2. Step 2: Analyze example array [1, 2, 3, 4]

   No element appears more than 2 times (half of 4). The algorithm will return a candidate but it is not a valid majority element.

3. Final Answer:

   An incorrect majority element -> Option A

4. Quick Check:

   No majority means algorithm returns wrong candidate [OK]

Hint: Algorithm assumes majority exists; else result may be wrong [OK]

Common Mistakes:

• Assuming algorithm returns null if no majority
• Thinking it returns highest frequency element always
• Believing it returns first element by default