nth_element() O(n)复杂度求第k+1小元素

nth_element() O(n)复杂度求第k+1小元素

函数原型

void nth_element(_RAIter, _RAIter, _RAIter);
void nth_element(_RAIter, _RAIter, _RAIter, _Compare);
void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth,
		_RandomAccessIterator __last)
void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth,
		_RandomAccessIterator __last, _Compare __comp)

先看官方的声明

void nth_element(_RAIter, _RAIter, _RAIter);

/*

@brief Sort a sequence just enough to find a particular position

using a predicate for comparison.

@ingroup sorting_algorithms

@param __first An iterator.

@param __nth Another iterator.

@param __last Another iterator.

@param __comp A comparison functor.

@return Nothing.

Rearranges the elements in the range @p [__first,__last) so that @p *__nth is the same element that would have been in that position had the whole sequence been sorted. The elements either side of @p *__nth are not completely sorted, but for any iterator @e i in the range @p [__first,__nth) and any iterator @e j in the range @p [__nth,__last) it holds that @p__comp(*j,*i) is false.

*/

请注意这句话

so that @p *__nthis the same element that would have been in that position had the whole sequence been sorted.

函数作用后*_nth的位置的值 与 排序之后*nth的值相等

那么我们要求数组a中第k小的值，第二个参数就应该填a+k-1

#include<algorithm>
#include<iostream>
using namespace std;
int main()
{
    int a[]={1,5,6,7,50,0,3,6,98,4,6,57,17,71,2,100};
    vector<int>aa(a,a+16);
    sort(aa.begin(),aa.end());
    for(auto i:aa)
        cout<<i<<" ";
    cout<<endl;
    nth_element(a,a+10,a+16);//a[10]的位置的值是排序之后a[10]的值 即第10+1小的位置
    for(auto i:a)
        cout<<i<<" ";
    cout<<endl;
}

或者可以写成一个函数

#include<algorithm>
#include<iostream>
using namespace std;
int GetNthVal(int *first,int *last,int k)/*第一小是最小，作用:求[first,last)中第k小的值*/
{
    if(k>last-first)
        return -1;
    nth_element(first,first+k-1,last);
    return first[k-1];
}
int main()
{
    int a[]={1,5,6,7,50,0,3,6,98,4,6,57,17,71,2,100};
    vector<int>aa(a,a+16);
    sort(aa.begin(),aa.end());
    for(auto i:aa)
        cout<<i<<" ";
    cout<<endl;
    cout<<GetNthVal(a,a+16,15);//求数组a中 第15小的值
}