partial_sort,partial_sort_copy,nth_element

直接代码：

//局部排序  partial_sort partial_sort_copy
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<functional>
using namespace std;
int main(){
	int a[]={1,4,7,2,5,8,3,6,9,10};
	vector<int>v(a,a+10);
	vector<int>vv(a,a+10);
	vector<int>vvv(10);
	partial_sort(v.begin(),v.begin()+4,v.end());//部分排序
	for(vector<int>::iterator itera=v.begin();itera!=v.end();++itera){
		cout<<*itera<<" ";
	}
	cout<<endl;
	//partial_sort(v.begin(),v.end(),v.end());则部分排序退化为普通排序，但是比sort要慢
	partial_sort(v.begin()+4,v.end(),v.end(),greater<int>());//前面四个从小到大，后面六个从大到小
	copy(v.begin(),v.end(),ostream_iterator<int>(cout," "));
	cout<<endl;

	partial_sort_copy(vv.begin(),vv.begin()+4,vvv.begin(),vvv.begin()+4);
	for(vector<int>::iterator it1=vvv.begin();it1!=vvv.begin()+4;++it1){
		cout<<*it1<<" ";
	}
	cout<<endl;

	partial_sort_copy(vv.begin()+4,vv.end(),vvv.begin()+4,vvv.end(),greater<int>());
	for(vector<int>::iterator it2=vvv.begin();it2!=vvv.end();++it2){
		cout<<*it2<<" ";
	}
	cout<<endl;
	
	system("pause");
	return 0;
}

//nth_element()
#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<functional>
using namespace std;
int main(){
	vector<int>v;
	for(int i=3;i<8;++i){
		v.push_back(i);
	}
	for(int j=2;j<7;++j){
		v.push_back(j);
	}
	for(int z=1;z<6;++z){
		v.push_back(z);
	}
	nth_element(v.begin(),v.begin()+3,v.end());
	cout<<"the four lowest elements are:";//前四个最小的
	copy(v.begin(),v.begin()+4,ostream_iterator<int>(cout," "));
	cout<<endl;

	nth_element(v.begin(),v.end()-4,v.end());
	cout<<"the four highest elements are:";//后四个最大的
	copy(v.end()-4,v.end(),ostream_iterator<int>(cout," "));
	cout<<endl;

	nth_element(v.begin(),v.begin()+3,v.end(),greater<int>());
	cout<<"the four highest elements are:";//前四个最大的
	copy(v.begin(),v.begin()+4,ostream_iterator<int>(cout," "));
	cout<<endl;
	system("pause");
	return 0;
}