Sort Colors Two Pointer Dutch Flag in DSA C - Build from Scratch

Sort Colors Using Two Pointer Dutch Flag Algorithm

📖 Scenario: You are working on a simple image processing tool that needs to sort pixels by their color codes. The colors are represented by numbers 0, 1, and 2, where 0 means red, 1 means white, and 2 means blue.To optimize the sorting, you will use the Dutch National Flag algorithm which sorts the colors in one pass using two pointers.

🎯 Goal: Build a program that sorts an array of colors represented by 0s, 1s, and 2s using the two-pointer Dutch Flag algorithm.

📋 What You'll Learn

Create an integer array called colors with the exact values: 2, 0, 2, 1, 1, 0

Create two integer variables called low and high to track positions

Implement the two-pointer Dutch Flag sorting logic using a while loop and a current index

Print the sorted array elements separated by spaces

💡 Why This Matters

🌍 Real World

Sorting colors efficiently is useful in image processing and graphics where pixels need to be grouped by color quickly.

💼 Career

Understanding two-pointer algorithms and in-place sorting is important for software engineering roles involving performance optimization and data manipulation.

1

Create the colors array

Create an integer array called colors with these exact values: 2, 0, 2, 1, 1, 0.

DSA C

# Your code here

Hint

Use curly braces to initialize the array with the exact numbers.

2

Create pointers for sorting

Create two integer variables called low and high. Set low to 0 and high to 5 (the last index of the array). Also create an integer variable current and set it to 0.

DSA C

int colors[] = {2, 0, 2, 1, 1, 0};
# Your code here

Hint

Remember array indexes start at 0, so last index is 5 for 6 elements.

3

Implement the Dutch Flag sorting logic

Use a while loop with the condition current <= high. Inside the loop, use if statements to check the value at colors[current]. If it is 0, swap it with colors[low], then increment low and current. If it is 2, swap it with colors[high] and decrement high. If it is 1, just increment current.

DSA C

int colors[] = {2, 0, 2, 1, 1, 0};
int low = 0;
int high = 5;
int current = 0;
// Your code here

Hint

Swap elements when you find 0 or 2, move pointers accordingly, and move current forward for 1.

4

Print the sorted colors array

Use a for loop to print each element of the colors array separated by spaces on a single line.

DSA C

int colors[] = {2, 0, 2, 1, 1, 0};
int low = 0;
int high = 5;
int current = 0;

while (current <= high) {
    if (colors[current] == 0) {
        int temp = colors[low];
        colors[low] = colors[current];
        colors[current] = temp;
        low++;
        current++;
    } else if (colors[current] == 2) {
        int temp = colors[high];
        colors[high] = colors[current];
        colors[current] = temp;
        high--;
    } else {
        current++;
    }
}
// Your code here

Hint

Use a loop to print all elements with spaces, then print a newline.