Java quick sorting tips and precautions

Key tips and precautions for mastering Java quick sort

Quick Sort (Quick Sort) is a commonly used sorting algorithm. Its core idea is to select a benchmark elements, divide the sequence to be sorted into two independent parts, one part has all elements smaller than the reference element, and the other part has all elements greater than the reference element, and then recursively sorts the two parts to finally get the ordered sequence.

Although the time complexity of quick sort is O(nlogn) in the average case, it will degenerate to O(n^2) in the worst case, so in actual use, we need to master some keys Tips and considerations to make quick sort more efficient.

1. Choose appropriate reference elements:
   The efficiency of quick sort is affected by the selection of reference elements. Usually, we can select the first element of the sequence to be sorted as the reference element, but if the sequence to be sorted is already close to ordered, this selection method may lead to the worst case scenario of degenerating to O(n^2). In order to avoid this situation, you can randomly select the reference element or select the middle element in the sequence to be sorted as the reference element.
2. Dividing subsequences:
   During the dividing process of quick sort, we need to divide the sequence to be sorted into two subsequences. You can use double pointers, starting from both ends of the sequence to be sorted, and continuously moving the pointers toward the middle until they meet, while exchanging the positions of elements smaller than the base element and elements larger than the base element. Finally, insert the base element into the meeting position to complete a division.
3. Recursive call:
   After the division is completed, we get two new subsequences. At this time, it is necessary to recursively call quick sort on the two subsequences to obtain the final ordered sequence. The end condition of the recursive call is that the length of the subsequence is less than or equal to 1.

The following is a sample code to demonstrate how to implement quick sort:

public class QuickSort {
    public static void quickSort(int[] arr, int low, int high) {
        if (low < high) {
            int pivot = partition(arr, low, high);
            quickSort(arr, low, pivot - 1);
            quickSort(arr, pivot + 1, high);
        }
    }

    public static int partition(int[] arr, int low, int high) {
        int pivot = arr[low];
        while (low < high) {
            while (low < high && arr[high] >= pivot) {
                high--;
            }
            arr[low] = arr[high];
            while (low < high && arr[low] <= pivot) {
                low++;
            }
            arr[high] = arr[low];
        }
        arr[low] = pivot;
        return low;
    }

    public static void main(String[] args) {
        int[] arr = {5, 3, 2, 1, 4};
        quickSort(arr, 0, arr.length - 1);
        for (int num : arr) {
            System.out.print(num + " ");
        }
    }
}

In the above code, we define the quickSort method to complete quick sorting Sort. Inside the method, we first select the first element of the sequence to be sorted as the base element, and divide it by calling the partition method. The partition method uses double pointers, starting from both ends of the sequence, moving the pointers toward the middle until they meet, and swapping the positions of elements smaller than the base element and elements larger than the base element. Finally, insert the base element where it meets.

In the main method, we tested the quick sort algorithm. The output result is 1 2 3 4 5, indicating that the sorting is correct.

By mastering the above key skills and precautions, we can better understand and apply the quick sort algorithm, thereby improving the efficiency and accuracy of sorting. At the same time, in actual development, we can further optimize the algorithm, such as using the three-number method to select benchmark elements to avoid the worst case scenario. In short, quick sort is a very practical and efficient sorting algorithm that is worth mastering and in-depth study.

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