Quick Sort Algorithm in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap two elements in the array.

DSA Java

int temp = arr[[1]];
arr[[1]] = arr[j];
arr[j] = temp;

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

Apivot

Bj

Ci

Dlow

Attempts:

3 left

💡 Hint

Common Mistakes

Using the pivot index instead of i or j.

Swapping with the wrong index causing incorrect partition.

We swap the element at index i with the element at index j to rearrange elements around the pivot.

2fill in blank

medium

Complete the code to choose the pivot element for partitioning.

DSA Java

int pivot = arr[[1]];

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Ahigh

Blow

C0

Dmid

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the first element as pivot without adjusting the algorithm.

Using an index outside the current sub-array.

The pivot is commonly chosen as the last element in the current sub-array, which is at index high.

3fill in blank

hard

Fix the error in the recursive quickSort call to sort the left partition.

DSA Java

quickSort(arr, low, [1]);

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ApivotIndex + 1

BpivotIndex

Chigh

DpivotIndex - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using pivotIndex instead of pivotIndex - 1 causes infinite recursion.

Using pivotIndex + 1 sorts the right partition instead.

The left partition ends at pivotIndex - 1 because pivotIndex is the pivot's final position.

4fill in blank

hard

Fill both blanks to complete the partition method's loop condition and increment.

DSA Java

for (int j = low; j [1] high; j[2]) {
    if (arr[j] <= pivot) {
        i++;
        // swap arr[i] and arr[j]
    }
}

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A<=

B<

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' causes the loop to include the pivot element.

Using '--' decrements j causing an infinite loop.

The loop runs while j is less than high, and j increments by 1 each iteration.

5fill in blank

hard

Fill all three blanks to complete the quickSort recursive calls and base case check.

DSA Java

if (low [1] high) {
    int pivotIndex = partition(arr, low, high);
    quickSort(arr, low, [2]);
    quickSort(arr, [3], high);
}

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

A<

BpivotIndex - 1

CpivotIndex + 1

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' in base case causes unnecessary recursion.

Swapping pivotIndex - 1 and pivotIndex + 1 in recursive calls causes wrong sorting.

The base case is when low is less than high. Then quickSort is called on left (low to pivotIndex-1) and right (pivotIndex+1 to high) partitions.

Practice

1. What is the main idea behind the Quick Sort algorithm?

easy

A. Insert elements one by one into their correct position

B. Compare every element with every other element and swap if needed

C. Merge two sorted arrays into one sorted array

D. Divide the array into smaller parts using a pivot and sort them recursively

Solution

1. Step 1: Understand Quick Sort's approach

   Quick Sort picks a pivot element and divides the array into parts smaller and larger than the pivot.

2. Step 2: Recursive sorting of parts

   It then sorts these parts recursively, combining them to get the sorted array.

3. Final Answer:

   Divide the array into smaller parts using a pivot and sort them recursively -> Option D

4. Quick Check:

   Quick Sort = Divide and conquer [OK]

Hint: Remember: Quick Sort uses a pivot to split and sort recursively [OK]

Common Mistakes:

• Confusing Quick Sort with Bubble Sort
• Thinking Quick Sort merges arrays like Merge Sort
• Assuming Quick Sort inserts elements one by one

2. Which of the following is the correct way to declare the partition method header in Java for Quick Sort?

easy

A. int partition(int arr[], low, high)

B. public int partition(int[] arr, int low, int high)

C. void partition(int[] arr, int low, int high)

D. public static partition(int[] arr, int low, int high)

Solution

1. Step 1: Check Java method declaration syntax

   Java methods must specify access modifier, return type, method name, and parameter types with names.

2. Step 2: Validate each option

   public int partition(int[] arr, int low, int high) correctly declares a public method returning int with parameters typed and named properly.

3. Final Answer:

   public int partition(int[] arr, int low, int high) -> Option B

4. Quick Check:

   Java method syntax = public int partition(int[] arr, int low, int high) [OK]

Hint: Remember Java method syntax: access + return type + name + parameters [OK]

Common Mistakes:

• Omitting return type
• Missing parameter types
• Incorrect order of parameters

3. What will be the output array after the first partition call on arr = [8, 3, 7, 6, 2] with pivot as last element?

medium

A. [2, 3, 6, 7, 8]

B. [3, 2, 6, 7, 8]

C. [2, 3, 7, 6, 8]

D. [8, 3, 7, 6, 2]

Solution

1. Step 1: Choose pivot and partition

   Pivot is last element 2. Move elements smaller than 2 to left.

2. Step 2: Rearrange array

   No elements smaller than 2, so swap 2 with first element 8. Result: [2, 3, 6, 7, 8]

3. Step 3: Check order after partition

   Elements less than pivot (none) are left, others right. No further swaps in first partition.

4. Final Answer:

   [2, 3, 6, 7, 8] -> Option A

5. Quick Check:

   Pivot 2 placed correctly, smaller left, larger right [OK]

Hint: Pivot is last element; smaller elements go left [OK]

Common Mistakes:

• Not swapping pivot correctly
• Assuming all smaller elements are sorted immediately
• Confusing pivot position after partition

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

int partition(int[] arr, int low, int high) {
  int pivot = arr[high];
  int i = low;
  for (int j = low; j < high; j++) {
    if (arr[j] < pivot) {
      swap(arr, i, j);
      i++;
    }
  }
  swap(arr, i, high);
  return i;
}

medium

A. Pivot should be arr[low] instead of arr[high]

B. Loop condition should be j <= high

C. Variable i should start at low - 1

D. Swap should happen only if arr[j] <= pivot

Solution

1. Step 1: Analyze initialization of i

   In Lomuto partition, i starts at low - 1 to track smaller elements index before loop.

2. Step 2: Check loop and swaps

   Starting i at low causes incorrect swaps and pivot placement.

3. Final Answer:

   Variable i should start at low - 1 -> Option C

4. Quick Check:

   i = low - 1 for correct partition [OK]

Hint: i starts one before low to track smaller elements [OK]

Common Mistakes:

• Starting i at low causes wrong partition
• Changing pivot position incorrectly
• Using wrong loop bounds

5. Given an array with many duplicate elements, which modification to Quick Sort improves performance most?

hard

A. Use a three-way partitioning to separate less, equal, and greater elements

B. Always pick the first element as pivot

C. Use Bubble Sort instead for duplicates

D. Skip sorting duplicates by checking each element

Solution

1. Step 1: Understand problem with duplicates

   Standard Quick Sort performs poorly with many duplicates due to repeated swaps.

2. Step 2: Apply three-way partitioning

   Three-way partitioning divides array into less than, equal to, and greater than pivot, reducing unnecessary work.

3. Step 3: Evaluate other options

   Picking first element or skipping sorting duplicates doesn't improve performance reliably; Bubble Sort is inefficient.

4. Final Answer:

   Use a three-way partitioning to separate less, equal, and greater elements -> Option A

5. Quick Check:

   Three-way partitioning = better with duplicates [OK]

Hint: Use three-way partitioning for arrays with many duplicates [OK]

Common Mistakes:

• Picking fixed pivot without randomization
• Ignoring duplicates in partitioning
• Switching to inefficient sorts unnecessarily