1. nth_element( b, n, e )         對n位置的元素進行比較，大的放後面，小的放前面，n必須是迭代器指向輸入的元素；

2. nth_element(b, n, e, p )

3.對比：partition() 演算法

#include <iostream>
#include <algorithm>
#include <iterator>
#include <vector>
#include <list>
#include <deque>
#include <functional>

using namespace std;

template<typename T>
void Print(const T& t)
{
	for(typename T::const_iterator itr=t.begin(); itr!=t.end(); ++itr)
	{
		cout<<*itr<<' ';
	}cout<<endl;
}

int main( int argc, char** argv )
{
	deque<int> deq;

	for(int i=3; i<=7; ++i)
	{
		deq.push_back(i);
	}
	for(int i=2; i<=6; ++i)
	{
		deq.push_back(i);
	}
	for(int i=1; i<=5; ++i)
	{
		deq.push_back(i);
	}
	Print(deq);

	nth_element(deq.begin(), deq.begin()+3, deq.end());
	Print(deq);
	copy(deq.begin(), deq.begin()+4, ostream_iterator<int>(cout, " "));
	cout<<endl;

	nth_element(deq.begin(), deq.end()-4, deq.end());
	copy(deq.end()-4, deq.end(), ostream_iterator<int>(cout, " "));
	cout<<endl;

	nth_element(deq.begin(), deq.begin()+3, deq.end(), greater<int>());
	copy(deq.begin(), deq.begin()+4, ostream_iterator<int>(cout, " "));
	cout<<endl;

	deque<int>::iterator pos;
	pos = partition(deq.begin(), deq.end(), bind2nd(less_equal<int>(), 3));
	copy(deq.begin(), pos, ostream_iterator<int>(cout, " "));

	return 0;
}