Quick sort【1月19日学习笔记】

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//Quick sort
#include<iostream>
using namespace std;

int partition(int A[],int start,int end) {
	int pivot = A[end];//默认选取末尾为主元
	int pIndex = start;//分区索引初始化
	for (int i = start; i < end; i++) {//从索引start开始扫描
		if (A[i] < pivot) {
			int temp = A[i];
			A[i] = A[pIndex];
			A[pIndex] = temp;
			pIndex++;
		}//比主元小的元素全交换在分区索引左边，分区索引右移则A[pIndex]比A[end]大（或相等）
	}
	int temp = A[end];
	A[end] = A[pIndex];
	A[pIndex] = temp;//交换后分区索引右边全比主元大或相等
	return pIndex;
}

int randomisedpartition(int A[], int start, int end) {
	srand(time(0));
	int random = (rand() % (end - start + 1)) + start;
	int temp = A[end];
	A[end] = A[random];
	A[random] = temp;//start与end间随机抽取元素作为新end
	return partition(A, start, end);//返回分区索引
}


void quicksort(int A[], int start,int end) {//传入索引起止参数，就地排序
	if (start >= end) return;//中止条件
	int pIndex = randomisedpartition(A, start, end);//建立分区并返回分区索引
	quicksort(A, start, pIndex - 1);//分区索引左侧排序
	quicksort(A, pIndex+1,end);//分区索引右侧排序
}//not stable
 //average case time complexity：O(nlogn)  by using random pIndex
 //worst case time complexity：O(n^2)
 //average case space complexity：O(logn)  by using random pIndex
//worst case space complexity：O(n)

int main() {
	int A[] = { 2,7,4,1,5,3 };
	quicksort(A, 0,5);
	for (int i = 0; i < 6; i++)  cout << A[i] << " ";
	cout << endl;
}