快速排序（蓝桥杯18E）

#include<iostream>
#include<cstdlib>
using namespace std;
int quick_select(int a[],int left,int right,int k) {
	int p = rand() % (right -left + 1) + left;
	int x = a[p];
	int i = left, j = right;
	while (i<j) {
		while (i<j && a[i]<x) {
			i++;
		}
		if (i<j) {
			a[j] = a[i];
			j--;
		}
		while (i<j&&a[j]>x) {
			j--;
		}
		if (i<j) {
			a[i] = a[j];
			i++;
		}
	}
	a[i] = x;
	p = i;
	if (i-left+1==k) {
		return a[i];
	}
	if (i-left+1<k) {
		return quick_select(a,i+1,right,k-(i-left+1));
	}
	else {
		return quick_select(a,left,i-1,k);
	}
}
int main() {
	int a[] = { 1,4,2,8,5,7,23,58,16,27,55,13,26,24,12 };
	int NUM_5 = quick_select(a,0,14,5);
	cout << NUM_5;
	return 0;
}

#include<iostream>
using namespace std;
void Qsort(int arr[],int L,int R) {
	if (L>=R) {
		return;
	}
	int left = L, right = R,key = arr[L],flag = 0;
	while (left < right) {
		if (flag == 0) {
			if (arr[right] < key) {
				arr[left] = arr[right];
				left++;
				flag = 1;
			}
			else {
				right--;
			}
		}
		else if (flag == 1) {
			if (arr[left]>key) {
				arr[right] = arr[left];
				right--;
				flag = 0;
			}
			else {
				left++;
			}
		}
	}
	arr[left] = key;
	Qsort(arr,L,left-1);
	Qsort(arr,left+1,R);
}
int main() {
	int arr[] = {7,6,3,4,10,-3,8,1,2,9,5,0};
	Qsort(arr,0,sizeof(arr)/sizeof(int)-1);
	for (int i = 0; i < sizeof(arr) / sizeof(int); i++) {
		cout << arr[i] << " ";
	}
	return 0;
}