數據結構與算法-C語言描述-三數取中快速排序（遞歸+非遞歸）+快速選擇

最近讀了《數據結構與算法-C語言描述》，受益匪淺，裏面的排序算法講的淺顯易懂，而且給出的快速排序算法代碼也非常簡潔，下面給出中快排的遞歸和非遞歸方式（參考了別人代碼實現的），同時將快排的變種快速選擇算法的代碼也附上，方便記憶。代碼我已經調試通過可以直接使用或者用來記憶用來面試。

#include<iostream>
#include<stack>
using namespace std;
#define cutoff (3)
//三數取中
int Median3(int A[], int left, int right) {
	int center = (left + right) / 2;
	if (A[left] > A[center])
		swap(A[left], A[center]);
	if (A[left] > A[right])
		swap(A[left], A[right]);
	if (A[center] > A[right])
		swap(A[center], A[right]);
	swap(A[center], A[right - 1]);
	return A[right - 1];
}
//插入排序
void InsertSort(int A[], int N) {
	int j, P;
	int tmp;
	for (P = 1; P < N; P++) {
		tmp = A[P];
		for (j = P; j > 0 && A[j - 1] > tmp; j--)
			A[j] = A[j - 1];
		A[j] = tmp;
	}
}
//快排遞歸版本
void Qsort(int A[], int left, int right) {
	int i, j;
	int pivot;
	if (left + cutoff <= right) {
		pivot = Median3(A, left, right);
		i = left;
		j = right - 1;
		for (;;){
			while (A[++i] < pivot) {}
			while (A[--j] > pivot) {}
			if (i < j)
				swap(A[i], A[j]);
			else
				break;
		}
		swap(A[i], A[right - 1]);
		Qsort(A, left, i - 1);
		Qsort(A, i + 1, right);
	}
	else
		InsertSort(A + left, right - left + 1);
}
//快速選擇遞歸版本 ：查找數組A中第K大的數字
void Qselect(int A[], int k, int left, int right) {
	int i, j;
	int pivot;
	if (left + cutoff <= right) {
		pivot = Median3(A, left, right);
		i = left;
		j = right - 1;
		for (;;){
			while (A[++i] < pivot) {};
			while (A[--j] > pivot) {};
			if (i < j)
				swap(A[i], A[j]);
			else
				break;
		}
		swap(A[i], A[right - 1]);
		if (k <= i)
			Qselect(A, k, left, i - 1);
		else if (k > i + 1)
			Qselect(A, k, i + 1, right);
	}
	else
		InsertSort(A + left, right - left + 1);
}
//快排非遞歸版本
void QuickSortNotRNew(int* A, int left, int right)
{
	int i, j;
	int pivot;
	stack<int> s;
	s.push(left);
	s.push(right);//後入的right，所以要先拿right
	while (!s.empty())//棧不爲空
	{
		int right = s.top();
		s.pop();
		int left = s.top();
		s.pop();
		if (left + cutoff <= right) {
			pivot = Median3(A, left, right);
			i = left;
			j = right - 1;
			for (;;){
				while (A[++i] < pivot) {};
				while (A[--j] > pivot) {};
				if (i < j)
					swap(A[i], A[j]);
				else
					break;
			}
			swap(A[i], A[right - 1]);
			int index = i;
			if ((index - 1) > left)//左子序列
			{
				s.push(left);
				s.push(index - 1);
			}
			if ((index + 1) < right)//右子序列
			{
				s.push(index + 1);
				s.push(right);
			}
		}
		else
			InsertSort(A + left, right - left + 1);
	}
}

int main() {
	int i;
	int a[] = { 10, 3, 6, 40, 1, 4, 9, 60, 30, 5, 2, 20, 50 };
	int b[] = { 10, 3, 6, 40, 1, 4, 9, 60, 30, 5, 2, 20, 50 };
	int ilen = (sizeof(a)) / (sizeof(a[0]));
	int range = 3;
	cout << "before sort:";
	for (i = 0; i<ilen; i++)
		cout << a[i] << " ";
	cout << endl;

	Qsort(a, 0, ilen - 1);
	cout << "after  sort:";
	for (i = 0; i<ilen; i++)
		cout << a[i] <<endl;

	QuickSortNotRNew(b, 0, ilen - 1);
	cout << "after Not recursive sort:";
	for (i = 0; i<ilen; i++)
		cout << b[i] <<endl;

	Qselect(a, range - 1 , 0, ilen - 1);
	cout << "the "<<range<<"th large number is:";
	cout << a[range - 1] << endl;
	return 0;
}

代碼運行結果

參考：《數據結構與算法-C語言描述》
https://blog.csdn.net/qq_36528114/article/details/78667034