寻找第n大的数 o(n)

//by [email protected]
#include<iostream>
#include<cstdlib>
#include<ctime>
using std::endl;
using std::cin;
using std::cout;
int partition(int *a,int l,int r);
int select(int *a,int l,int r,int i);
void swap(int *a,int i,int j);
int main(){
	srand(time(0));
	int count;
	cout<<"请输入数组个数:";
	cin>>count;
	int *a = new int[count];
	cout<<"请输入数组：";
	for(int i = 0;i<count;i++)
		cin>>a[i];
	cout<<"请输入希望寻找第几大的数:";
	int ch;
	cin>>ch;
	cout<<"it is :"<<select(a,0,count-1,ch);
	cout<<endl;
	return 0;
}
int select(int *a ,int l,int r,int i){
	int re = partition(a,l,r);
	if(r == l)
		return a[r];
	if(re -l+1 >= i){
		select(a,l,re,i);
	}else{
		select(a,re+1,r,i-(re-l+1));
	}
}
void swap(int *a,int i,int j){
	int temp = a[i];
	a[i] = a[j];
	a[j] = temp;
}
int partition(int *a,int l,int r){
	if((l-r)==0)
		return l;
	int m = l+rand()%(r-l);
	swap(a,m,r);
	int left = l-1;
	int right = l-1;
	while(++right < r){
		if(a[right] < a[r]){
			swap(a,right,++left);
		}	
	}
	swap(a,left+1,r);
	return left+1;
}