快速排序及改进

#include<bits/stdc++.h>
using namespace std;
int Partition(int a[],int l,int r) //原版
{
	int k=a[l];
	while(l<r)
	{
		while(l<r&&a[r]>=k)r--;
		a[l]=a[r];
		while(l<r&&a[l]<=k)l++;
		a[r]=a[l];
	}
	a[l]=k;
	return l;
}
int Partition2(int a[],int l,int r)//改进版,即取l、mid、r对应值中的中间值作为数轴元素 
{
	int mid=(l+r)/2;
	int k1=a[l],k2=a[mid],k3=a[r];
	int k;
	if(k1>=min(k2,k3)&&k1<=max(k2,k3))
	{
		k=k1;
	}
	else if(k3>=min(k2,k1)&&k3<=max(k2,k1))
	{
		k=k3;
		while(l<r&&a[l]<=k)l++;
		a[r]=a[l];
	}
	else
	{
		k=k2;
		while(l<r&&a[l]<=k)l++;
		a[mid]=a[l];
	}

	while(l<r)
	{
		while(l<r&&a[r]>=k)r--;
		a[l]=a[r];
		while(l<r&&a[l]<=k)l++;
		a[r]=a[l];
	}
	a[l]=k;
	return l;
}
void QuickSort(int a[],int l,int r)
{
	if(l>=r)return;
	int p=Partition2(a,l,r);
	QuickSort(a,l,p-1);
	QuickSort(a,p+1,r); 
}

int main()
{
	int a[]={5,3,4,1,2};
	QuickSort(a,0,4);
	for(int i=0;i<5;++i)
		cout<<a[i]<<" "; 
}