交換排序之快速排序

package com.sort;

//快速排序：平均（O(Nlog n)）,最差：O（n^2）
public class QuickSort {
	public static void main(String[] args) {
		int[] a = new int[]{6,5,4,3,2,1};
		quickSort(a,0,a.length-1);
		for(int array:a){
			System.out.println(array);
		}
	}

	private static void quickSort(int[] a,int start, int end) {
		if(start >= end){
			return;
		}
		int divider = partition(a,start,end);
		quickSort(a, start, divider-1);
		quickSort(a, divider+1, end);
	}

	private static int partition(int[] a, int start, int end) {
		int elem = a[end];//以最右側的元素作爲主元
		int cursor = start-1;//光標，指向存放小於elem的位置
		for(int i=start; i<end; i++){
			if(a[i]<=elem){
				cursor++;
				swap(a,cursor,i);
			}
		}
		swap(a,cursor+1,end);
		return cursor+1;
	}
	
	private static void swap(int[] a, int j, int i) {
		int temp = a[j];
		a[j] = a[i];
		a[i] = temp;
	}
}