分治法查找 第k小元素

#include<bits/stdc++.h>
using namespace std;

int Partition(int a[], int left, int right) {		//一次快排 
	int pivot = a[left];
	while (left < right) {
		while (left<right && a[right]>pivot) {
			right--;
		}
		a[left] = a[right];
		while (left < right && a[left] <= pivot) {
			left++;
		}
		a[right] = a[left];
	}
	a[left] = pivot;
	return left;
}

void QuickSort(int a[], int left, int right) {	//整体快排
	if (left > right) return;
	if (left < right) {
		int p = Partition(a, left, right);
		QuickSort(a, left, p - 1);
		QuickSort(a, p + 1, right);
	}
}

void swap_a(int a[], int x, int y) {	//交换a[x]与a[y]的值
	int temp = a[x];
	a[x] = a[y];
	a[y] = temp;
}

//注意用数组下标，因为快排是用数组下标
int select_min_k(int a[], int left, int right, int k) {

	do {
		int j = rand() % (right - left + 1) + left;
		swap_a(a, left, j);
		j = Partition(a, left, right);
		if (k == j + 1) {
			return j;
		}
		else if (k < j + 1) {
			right = j;
		}
		else {
			left = j + 1;
		}
	} while (1);
}


int main() {
	int N;
	cin >> N;
	int a[10000] = {};
	int i, j, temp;
	for (i = 0; i < N; i++) {
		cin >> a[i];
	}

	int k;
	cin >> k;
	if (k<1 || k>N) {
		cout << "不存在第" << k << "小元素" << endl;
		return 0;
	}

	int location = select_min_k(a, 0, N - 1, k);
	cout << a[location] << endl;

	return 0;
}

测试结果：