Quick sort in Java

Principle of quick sort

Quick sort is an improvement on bubble sort. Bubble sort is to compare small values ​​one by one. One end, and the larger value is placed on the other end to achieve the purpose of sorting.

Quick sorting is to first select a critical value, put the values ​​smaller than the critical value on one end, and put the values ​​larger than the critical value on the other end. . Repeat the method in the previous paragraph, and you can divide the two sides that have passed the critical value and divide them twice... After sorting the data, the entire quick sort is completed.

Quick Sort Algorithm

Core Algorithm:

//QuickSort
while(i < j) {
		while(num[j] > tmp && j > i)
			--j;
		while(num[i] <= tmp && i < j) {
			++i;
		}
		if(i < j) {
			t = num[i];
			num[i] = num[j];
			num[j] = t;
		}
	}
	num[left] = num[i];
	num[i] = tmp;

The following is the complete QuickSort program:

//QuickSort.java
public class QuickSort {
	public static void main(String[] args) {
		int[] num = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
		
		System.out.print("Qriginal array is:");
		for (int i = 0; i < num.length; i++) {
			System.out.print(num[i] + " ");
		}
		System.out.println();
		
		//QuickSort
		quicksort(num, 0, 9);
		
		System.out.print("Sorted array is:");
		for (int i = 0; i < num.length; i++) {
			System.out.print(num[i] + " ");
		}
		System.out.println();
	}
	
	public static void quicksort(int[] num, int left, int right) {
		if(left > right)
			return;
		int tmp, i, j, t;
		tmp = num[left];
		i = left;
		j = right;
		while(i < j) {
			while(num[j] > tmp && j > i)
				--j;
			while(num[i] <= tmp && i < j) {
				++i;
			}
			if(i < j) {
				t = num[i];
				num[i] = num[j];
				num[j] = t;
			}
		}
		num[left] = num[i];
		num[i] = tmp;
		quicksort(num, left, i - 1);
		quicksort(num, i + 1, right);
	}
}

The program output is shown in the figure below:

Qriginal array is:10 9 8 7 6 5 4 3 2 1
Sorted array is:1 2 3 4 5 6 7 8 9 10

Quick sorting is more efficient than other sorting methods, so quick sorting is the best general sorting method now. The time complexity of QuickSort is O(nlogn).

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