How Does the QuickSort Partition Algorithm Work?

What is the QuickSort partition algorithm and how does it function?

int partition(int arr[], int low, int high) {
    int pivot = arr[high];
    int i = (low - 1);
    for (int j = low; j < high; j++) {
        if (arr[j] < pivot) {
            i++;
            swap(arr[i], arr[j]);
        }
    }
    swap(arr[i + 1], arr[high]);
    return (i + 1);
}

The QuickSort partition algorithm is a crucial component of the QuickSort sorting algorithm. It is responsible for selecting a 'pivot' element from the array and rearranging the other elements into two partitions, collecting all elements less than the pivot to one side and all elements greater to the other side. This leads to a more efficiently sorted array with a time complexity of O(n log n) in average cases.

int partition(int arr[], int low, int high) {
    int pivot = arr[high];  // Select the pivot
    int i = (low - 1); // Index of smaller element
    for (int j = low; j < high; j++) {
        // If current element is smaller than or equal to pivot
        if (arr[j] < pivot) {
            i++;
            swap(arr[i], arr[j]); // Swap elements
        }
    }
    swap(arr[i + 1], arr[high]); // Swap the pivot to correct position
    return (i + 1);

Causes

• The algorithm relies on selecting a pivot element, which impacts efficiency.
• An improper choice of pivot can lead to unbalanced partitions, affecting performance.

Solutions

• Implementing a random pivot selection mechanism to avoid worst-case scenarios.
• Optimizing the partitioning process to balance the elements during sorting.

Mistake: Choosing the first or last element as a pivot without considering the data distribution.

Solution: Consider using the median of three (first, middle, last) to choose a better pivot.

Mistake: Failing to handle duplicate values properly.

Solution: Modify the partitioning logic to account for duplicate values effectively.