[Algorithm] Sort Colors

Given an array nums with n objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue.

We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.

You must solve this problem without using the library's sort function.

 

Example 1:

Input: nums = [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Example 2:

Input: nums = [2,0,1]
Output: [0,1,2]

 

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is either 0, 1, or 2.

 

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

 

public class Solution {
    public void SortColors(int[] nums) {
        int red = 0; // Pointer for the next position where 0 should be placed
        int white = 0; // Pointer to iterate through the array
        int blue = nums.Length - 1; // Pointer for the next position where 2 should be placed
        
        while(white <= blue) { // Continue until all elements are sorted
            if(nums[white] == 0) { // If current element is 0
                // Swap current element with the element at left pointer
                wap(nums, red, white);
                // Move left pointer and current pointer to the right
                red++;
                white++;
            }
            else if (nums[white] == 1) { // If current element is 1 (white)
                // Move current pointer to the right
                white++;
            }
            else{ // If current element is 2
                // Swap current element with the element at right pointer
                Swap(nums, white, blue);
                // Move right pointer to the left
                blue--;
            }
        }
    }
    
    // Helper function to swap two elements in the array
    private void Swap(int[] nums, int i, int j)
    {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}