SELECT 算法

// main.cpp
// Find the N-th minimum value, based on Partition

#include <iostream>
#include <iterator>
#include <algorithm>
using namespace std ;

int Minimum_N(int arr[], int s, int e, int n)
{
    // Partition
    int j = s ;
    for (int i = s ;i < e-1 ;i++)
    {
        // Exchage two integer
        if (arr[i] < arr[e-1])
        {
            swap(arr[i], arr[j]) ;

            j++ ;
        }//if
    }//for
    swap(arr[j], arr[e-1]) ;

    ostream_iterator<int> out(cout, " ") ;
    copy(arr+s, arr+e, out) ;
    cout <<endl ;

    // Find Minimum_N
    if (n > j+1)
    {
        return Minimum_N(arr, j+1, e, n) ;
    }
    else if (n < j+1)
    {
        return Minimum_N(arr, s, j, n) ;
    }//if

    return arr[j] ;
}

int main()
{
    int arr[] = {3, 2, 6, 1, 9, 7, 0, 8, 4, 5} ;

    ostream_iterator<int> out(cout, " ") ;
    copy(arr, arr + sizeof(arr)/sizeof(arr[0]), out) ;
    cout <<endl ;

    cout <<"Minimum 2 : " <<Minimum_N(arr, 0, sizeof(arr)/sizeof(arr[0]), 2) <<endl ;

    return 0 ;
}

   不完全排序, 查找第 N 小的元素.