Quick Sort as Divide and Conquer in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to choose the pivot element as the last item in the array.

DSA Javascript

function quickSort(arr) {
  if (arr.length <= 1) return arr;
  const pivot = arr[1];
  // rest of the code
}

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

A[arr.length - 1]

B[0]

C[Math.floor(arr.length / 2)]

D[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the first element as pivot instead of the last.

Using an index out of bounds like arr[arr.length].

The pivot is chosen as the last element, so we use arr[arr.length - 1].

2fill in blank

medium

Complete the code to partition the array by comparing elements to the pivot.

DSA Javascript

function partition(arr, low, high) {
  const pivot = arr[high];
  let i = low - 1;
  for (let j = low; j < high; j++) {
    if (arr[j] [1] pivot) {
      i++;
      [arr[i], arr[j]] = [arr[j], arr[i]];
    }
  }
  [arr[i + 1], arr[high]] = [arr[high], arr[i + 1]];
  return i + 1;
}

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

A<=

B>

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or >= which reverses the partition logic.

Using == which only matches equal elements.

We move elements less than or equal to the pivot to the left side.

3fill in blank

hard

Fix the error in the recursive quickSort call to sort left and right parts.

DSA Javascript

function quickSort(arr, low = 0, high = arr.length - 1) {
  if (low < high) {
    const pi = partition(arr, low, high);
    quickSort(arr, low, [1]);
    quickSort(arr, pi + 1, high);
  }
  return arr;
}

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

Api + 1

Bhigh

Cpi - 1

Dpi

Attempts:

3 left

💡 Hint

Common Mistakes

Using pi instead of pi - 1 causes infinite recursion.

Using pi + 1 sorts the right side twice.

We sort the left part from low to pi - 1 because pi is the pivot index.

4fill in blank

hard

Fill both blanks to complete the partition function's swap logic correctly.

DSA Javascript

function partition(arr, low, high) {
  const pivot = arr[high];
  let i = low - 1;
  for (let j = low; j < high; j++) {
    if (arr[j] <= pivot) {
      i++;
      [arr[[1]], arr[[2]]] = [arr[[2]], arr[[1]]];
    }
  }
  [arr[i + 1], arr[high]] = [arr[high], arr[i + 1]];
  return i + 1;
}

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

Ai

Bj

Clow

Dhigh

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping wrong indices causes incorrect partitioning.

Using high or low instead of i and j.

We swap elements at indices i and j to move smaller elements left.

5fill in blank

hard

Fill both blanks to complete the quickSort function with default parameters and recursive calls.

DSA Javascript

function quickSort(arr, [1] = 0, [2] = arr.length - 1) {
  if ([1] < [2]) {
    const pi = partition(arr, [1], [2]);
    quickSort(arr, [1], pi - 1);
    quickSort(arr, pi + 1, [2]);
  }
  return arr;
}

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

Alow

Bhigh

Cpi

Dpivot

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names causes confusion.

Mixing up pi with parameters.

The parameters are low and high with default values. The recursive calls use low and high accordingly.

Practice

1. What is the main idea behind the Quick Sort algorithm in the divide and conquer approach?

easy

A. Divide the array into smaller parts, sort them, and combine the results

B. Sort the array by repeatedly swapping adjacent elements

C. Use extra space to merge two sorted arrays

D. Sort the array by selecting the smallest element each time

Solution

1. Step 1: Understand divide and conquer in Quick Sort

   Quick Sort divides the array into parts based on a pivot, sorts each part recursively.

2. Step 2: Compare with other sorting methods

   Unlike bubble or selection sort, Quick Sort uses division and recursion, not just swapping or selecting smallest.

3. Final Answer:

   Divide the array into smaller parts, sort them, and combine the results -> Option A

4. Quick Check:

   Quick Sort = Divide and conquer [OK]

Hint: Remember: divide, sort parts, then combine [OK]

Common Mistakes:

• Confusing Quick Sort with bubble or selection sort
• Thinking Quick Sort merges arrays like merge sort
• Ignoring the pivot selection step

2. Which of the following is the correct syntax to choose a pivot element in a Quick Sort function in JavaScript?

easy

A. const pivot = arr.push();

B. const pivot = arr.length;

C. const pivot = arr.sort();

D. const pivot = arr[0];

Solution

1. Step 1: Identify how to select pivot in Quick Sort

   The pivot is usually an element from the array, often the first or last element.

2. Step 2: Check each option's correctness

   const pivot = arr[0]; selects the first element as pivot correctly. Others use wrong methods or properties.

3. Final Answer:

   const pivot = arr[0]; -> Option D

4. Quick Check:

   Pivot selection = arr[0] [OK]

Hint: Pivot is an element, not array length or method result [OK]

Common Mistakes:

• Using array length as pivot
• Calling array methods instead of accessing element
• Confusing pivot with sorted array

3. What will be the output of the following JavaScript Quick Sort partition step?

const arr = [4, 2, 7, 1];
const pivot = arr[0]; // 4
let left = [];
let right = [];
for(let i = 1; i < arr.length; i++) {
  if(arr[i] <= pivot) left.push(arr[i]);
  else right.push(arr[i]);
}
console.log(left);
console.log(right);

medium

A. [4, 2] and [7, 1]

B. [7] and [2, 1]

C. [2, 1] and [7]

D. [1, 2, 4] and []

Solution

1. Step 1: Identify pivot and partition logic

   Pivot is 4. Elements less or equal to 4 go to left, others to right.

2. Step 2: Partition the array

   Elements: 2 <= 4 -> left, 7 > 4 -> right, 1 <= 4 -> left. So left = [2,1], right = [7].

3. Final Answer:

   [2, 1] and [7] -> Option C

4. Quick Check:

   Partition by pivot 4 = [2,1] left, [7] right [OK]

Hint: Compare each element to pivot, separate into left/right [OK]

Common Mistakes:

• Including pivot in left or right arrays
• Mixing up less than and greater than conditions
• Forgetting to start loop from index 1

4. Identify the error in this Quick Sort partition code snippet:

function partition(arr) {
  const pivot = arr[0];
  let left = [];
  let right = [];
  for(let i = 0; i < arr.length; i++) {
    if(arr[i] < pivot) left.push(arr[i]);
    else right.push(arr[i]);
  }
  return [...left, pivot, ...right];
}

medium

A. Use arr[i] <= pivot in condition instead of <

B. Loop should start from 1 to avoid including pivot twice

C. Pivot should be last element, not first

D. Return statement should not use spread operator

Solution

1. Step 1: Analyze loop range and pivot usage

   The loop starts at 0, so pivot element arr[0] is included in left or right arrays again.

2. Step 2: Correct loop start index

   Loop should start at 1 to skip pivot element and avoid duplication.

3. Final Answer:

   Loop should start from 1 to avoid including pivot twice -> Option B

4. Quick Check:

   Loop index = 1 to exclude pivot [OK]

Hint: Start loop after pivot index to avoid duplicates [OK]

Common Mistakes:

• Starting loop at 0 causing pivot duplication
• Changing pivot position unnecessarily
• Misunderstanding spread operator usage

5. Given the array [3, 6, 8, 10, 1, 2, 1], what is the correct sorted array after applying Quick Sort fully?

hard

A. [1, 1, 2, 3, 6, 8, 10]

B. [1, 2, 3, 6, 8, 10]

C. [3, 6, 8, 10, 1, 2, 1]

D. [10, 8, 6, 3, 2, 1, 1]

Solution

1. Step 1: Understand Quick Sort fully sorts the array

   Quick Sort recursively partitions and sorts all parts until fully sorted.

2. Step 2: Sort the array manually

   Sorted array is [1, 1, 2, 3, 6, 8, 10], including duplicates in order.

3. Final Answer:

   [1, 1, 2, 3, 6, 8, 10] -> Option A

4. Quick Check:

   Quick Sort output = sorted array ascending [OK]

Hint: Quick Sort returns fully sorted array ascending [OK]

Common Mistakes:

• Ignoring duplicate elements
• Confusing sorted with original array
• Thinking Quick Sort reverses array