Sort Colors Two Pointer Dutch Flag in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Dutch Flag Algorithm on Mixed Colors

What is the output of the following code after sorting the colors using the Dutch Flag algorithm?

DSA Python

def sortColors(nums):
    low, mid, high = 0, 0, len(nums) - 1
    while mid <= high:
        if nums[mid] == 0:
            nums[low], nums[mid] = nums[mid], nums[low]
            low += 1
            mid += 1
        elif nums[mid] == 1:
            mid += 1
        else:
            nums[mid], nums[high] = nums[high], nums[mid]
            high -= 1

nums = [2, 0, 2, 1, 1, 0]
sortColors(nums)
print(nums)

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

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

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

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

💡 Hint

Think about how the pointers low, mid, and high move and swap elements.

The Dutch Flag algorithm sorts the array in one pass by moving 0s to the front, 2s to the end, and 1s stay in the middle.

🧠 Conceptual

intermediate

Number of Passes in Dutch Flag Algorithm

How many passes over the array does the Dutch Flag algorithm make to sort colors?

A Depends on the input size

B One pass

C Three passes

D Two passes

💡 Hint

Consider how the pointers move through the array.

The algorithm uses three pointers and sorts the array in a single pass by swapping elements as needed.

🔧 Debug

advanced

Identify the Bug in Dutch Flag Implementation

What error will this code produce when run?

DSA Python

def sortColors(nums):
    low, mid, high = 0, 0, len(nums) - 1
    while mid < high:
        if nums[mid] == 0:
            nums[low], nums[mid] = nums[mid], nums[low]
            low += 1
            mid += 1
        elif nums[mid] == 1:
            mid += 1
        else:
            nums[mid], nums[high] = nums[high], nums[mid]
            high -= 1

nums = [2, 0, 1]
sortColors(nums)
print(nums)

A IndexError

B [0, 1, 2]

C SyntaxError

D Infinite loop

💡 Hint

Check the loop condition carefully.

The loop condition should be 'mid <= high' to ensure all elements are processed. Using 'mid < high' can cause an infinite loop.

❓ Predict Output

advanced

Final Array State After Sorting

What is the final state of the array after running this code?

DSA Python

def sortColors(nums):
    low, mid, high = 0, 0, len(nums) - 1
    while mid <= high:
        if nums[mid] == 0:
            nums[low], nums[mid] = nums[mid], nums[low]
            low += 1
            mid += 1
        elif nums[mid] == 1:
            mid += 1
        else:
            nums[mid], nums[high] = nums[high], nums[mid]
            high -= 1

nums = [1, 2, 0, 1, 2, 0, 1]
sortColors(nums)
print(nums)

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

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

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

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

💡 Hint

Remember the order: all 0s first, then 1s, then 2s.

The algorithm sorts the array so that all 0s come first, then 1s, then 2s.

🧠 Conceptual

expert

Why Dutch Flag Algorithm is Optimal for Sorting Colors?

Why is the Dutch Flag algorithm considered optimal for sorting an array containing only 0s, 1s, and 2s?

A It sorts the array by counting each color and then overwriting

B It uses recursion to divide and conquer the array

C It sorts the array in one pass with constant extra space

D It uses built-in sort which is optimized for small ranges

💡 Hint

Think about time complexity and space usage.

The Dutch Flag algorithm sorts the array in a single pass and uses only a few pointers, so it uses constant extra space and linear time.