Sort Colors

Problem statement

Sort the array containing red, white, and blue colors (0, 1, 2) in-place without using the library's sort function. Use the Dutch National Flag algorithm.

Constraints

• n == nums.length

• 1 <= n <= 300

• nums[i] is either 0, 1, or 2

Examples

nums = [1, 0, 2, 1, 2, 1, 0, 2]

output = [0, 0, 1, 1, 1, 2, 2, 2]

Sudo Code

1. Set three pointers: low, mid, and high to the start, start, and end of the array, respectively

2. Loop while mid is less than or equal to high.

3. If the element at mid is 0, swap with low, increment both low and mid

4. If the element at mid is 2, swap with high, decrement high.

5. If the element at mid is 1, increment mid

Dry Run

Solution

public void sortColors(int[] nums) {
        int n = nums.length;

        // Step 1. Set three pointers: low, mid, and high to the start, start, and end of the array, respectively.
        int low = 0;
        int mid = 0;
        int high = n-1;

        // Step 2. Loop while mid is less than or equal to high.
        while(mid <= high){
            if(nums[mid] == 0){
                // Step 3. If the element at mid is 0, swap with low, increment both low and mid
                int temp = nums[mid];
                nums[mid] = nums[low];
                nums[low] = temp;
                mid++;
                low++;
            } else if(nums[mid] == 2){
                // Step 4. If the element at mid is 2, swap with high, decrement high.
                int temp = nums[mid];
                nums[mid] = nums[high];
                nums[high] = temp;
                high--;
            }else{
                // Step 5. If the element at mid is 1, increment mid.
                mid++;
            }
        }
    }

Time complexity Analysis

Time complexity - O(N)

Space Complexity - O(1)

Topics Covered

Two pointers

Companies

Amazon, Google, Microsoft, Apple, Adobe, Linkedln, Uber, Qualcomm