Sort Colors Two Pointer Dutch Flag in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the pointer for the next position of 0.

DSA Java

int low = [1];

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

A0

B1

C-1

Dnums.length

Attempts:

3 left

💡 Hint

Common Mistakes

Starting 'low' at 1 or -1 causes index errors.

Initializing 'low' at nums.length is out of bounds.

The 'low' pointer starts at index 0 to track where the next 0 should be placed.

2fill in blank

medium

Complete the code to swap the current element with the element at the 'low' pointer.

DSA Java

int temp = nums[i]; nums[i] = nums[[1]]; nums[low] = temp;

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

Ahigh

Blow

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with 'high' pointer moves wrong elements.

Swapping with 'i' does nothing.

Using 0 as index causes wrong swaps.

We swap nums[i] with nums[low] to move 0s to the front.

3fill in blank

hard

Fix the error in the loop condition to correctly iterate through the array.

DSA Java

while (i [1] high) {

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

A>=

B<=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' can cause index out of bounds if 'high' is the last index.

Using '>' or '>=' causes infinite loops or no iteration.

We use '<' to iterate until just before 'high' index, since 'high' is inclusive in the algorithm.

4fill in blank

hard

Fill both blanks to correctly update pointers after swapping 0 and 2.

DSA Java

if (nums[i] == 0) { swap(nums, i, [1]); [2]++; i++; }

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

Alow

Bhigh

Ci

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing 'high' instead of 'low' after swapping 0.

Not incrementing 'i' causes infinite loop.

After swapping with 'low', increment 'low' and 'i' to move forward.

5fill in blank

hard

Fill all three blanks to correctly handle swapping 2 and updating pointers.

DSA Java

if (nums[i] == 2) { swap(nums, i, [1]); [2]--; [3] = [3]; }

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

Alow

Bhigh

Ci

Di + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing 'i' after swapping 2 causes skipping elements.

Swapping with 'low' instead of 'high' is incorrect.

Swap with 'high', decrement 'high', and keep 'i' same to recheck swapped element.

Practice

1. What is the main goal of the Sort Colors Two Pointer Dutch Flag algorithm?

easy

A. To sort an array containing only 0s, 1s, and 2s in a single pass

B. To sort any array of integers in ascending order

C. To find the maximum element in an array

D. To reverse the elements of an array

Solution

1. Step 1: Understand the problem context

   The algorithm is designed specifically for arrays with only three distinct values: 0, 1, and 2.

2. Step 2: Identify the algorithm's purpose

   It sorts these values in one pass using two pointers, placing all 0s first, then 1s, then 2s.

3. Final Answer:

   To sort an array containing only 0s, 1s, and 2s in a single pass -> Option A

4. Quick Check:

   Sort Colors = Sort 0,1,2 in one pass [OK]

Hint: Remember: Dutch Flag sorts 0,1,2 in one pass [OK]

Common Mistakes:

• Thinking it sorts any integer array
• Confusing it with general sorting algorithms
• Assuming it reverses the array

2. Which of the following is the correct way to initialize the pointers for the Dutch Flag algorithm in Java?

easy

A. int low = 0, mid = 0, high = 0;

B. int low = arr.length - 1, mid = 0, high = 0;

C. int low = 0, mid = arr.length - 1, high = 0;

D. int low = 0, mid = 0, high = arr.length - 1;

Solution

1. Step 1: Identify pointer roles

   Low and mid start at the beginning (index 0), high starts at the end (last index).

2. Step 2: Match initialization with roles

   int low = 0, mid = 0, high = arr.length - 1; correctly sets low and mid to 0, and high to last index (arr.length - 1).

3. Final Answer:

   int low = 0, mid = 0, high = arr.length - 1; -> Option D

4. Quick Check:

   Pointers start low=0, mid=0, high=end [OK]

Hint: Low and mid start at 0, high at array end [OK]

Common Mistakes:

• Setting pointers all to zero
• Starting high pointer at 0 instead of end
• Confusing mid and high initial positions

3. What will be the output array after running the Dutch Flag algorithm on int[] arr = {2, 0, 2, 1, 1, 0};?

medium

A. [2, 2, 1, 1, 0, 0]

B. [0, 0, 1, 1, 2, 2]

C. [0, 1, 0, 1, 2, 2]

D. [1, 0, 2, 0, 1, 2]

Solution

1. Step 1: Apply Dutch Flag sorting logic

   Start with low=0, mid=0, high=5. Swap 2 with 0 at low, move pointers accordingly to group 0s, then 1s, then 2s.

2. Step 2: Final sorted array

   After sorting, the array becomes [0, 0, 1, 1, 2, 2].

3. Final Answer:

   [0, 0, 1, 1, 2, 2] -> Option B

4. Quick Check:

   Sorted colors = 0s, then 1s, then 2s [OK]

Hint: 0s first, then 1s, then 2s after sorting [OK]

Common Mistakes:

• Not swapping correctly
• Mixing order of 1s and 2s
• Assuming original order stays same

4. Identify the error in this Java code snippet for the Dutch Flag algorithm:

int low = 0, mid = 0, high = arr.length - 1;
while (mid <= high) {
  if (arr[mid] == 0) {
    swap(arr, low, mid);
    low++;
  } else if (arr[mid] == 1) {
    mid++;
  } else {
    swap(arr, mid, high);
    high--;
  }
}

medium

A. Swap function parameters are wrong

B. Incorrect condition in while loop

C. Missing increment of mid pointer after swapping 0

D. No error, code is correct

Solution

1. Step 1: Analyze pointer increments after swaps

   When swapping 0, mid pointer must also increment to avoid infinite loop.

2. Step 2: Identify missing mid++ after swapping 0

   Code increments low but forgets to increment mid after swapping 0, causing repeated processing of same element.

3. Final Answer:

   Missing increment of mid pointer after swapping 0 -> Option C

4. Quick Check:

   mid++ needed after swapping 0 [OK]

Hint: Always increment mid after swapping 0 [OK]

Common Mistakes:

• Forgetting to increment mid after swap
• Changing while loop condition incorrectly
• Swapping wrong indices

5. Given an array int[] arr = {1, 2, 0, 2, 1, 0, 0, 2, 1}, how many swaps will the Dutch Flag algorithm perform to sort it?

hard

A. 6

B. 4

C. 5

D. 7

Solution

1. Step 1: Trace swaps during sorting

   Initial array: [1,2,0,2,1,0,0,2,1]. Swaps occur when 0 or 2 are moved. Counting swaps:
   - Swap 1: mid=1 (2) with high=8
   - Swap 2: mid=2 (0) with low=0
   - Swap 3: mid=3 (2) with high=7
   - Swap 4: mid=3 (2) with high=6
   - Swap 5: mid=3 (0) with low=1
   - Swap 6: mid=5 (0) with low=2

2. Step 2: Total swaps counted

   Total swaps performed are 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Count swaps carefully during pointer moves [OK]

Hint: Count swaps when 0 or 2 move positions [OK]

Common Mistakes:

• Counting swaps only for 0s
• Ignoring swaps when swapping with same value
• Miscounting pointer moves as swaps