Select算法（最坏复杂度O(n)）

 #include<iostream>
 #include <stdio.h>
 #include <stdlib.h>
 #include <algorithm>
 #include <string>
 #include <string.h>
 using namespace std;

 const int nMax = ;
 int A[nMax+];
 int B[nMax+];//用来每次5分法后保存要比较的值在A中的下标
 int AIndex[nMax+]; //用来保存A的初始化下标

 //通过插入排序获取中位数下标
 int InsertSort(int A[], int B[], int start, int end)
 {
     if (start == end)
     {
         return B[start];
     }

     for (int i = start+; i <= end; ++i)
     {
         int num = A[B[i]];
         int j = i-;
         for ( ; j >= start; --j)
         {
             if (num < A[B[j]])
             {
                 A[B[j + ]] = A[B[j]];
             }
             else
             {
                 break;
             }
         }
         A[B[j + ]] = num;
     }

     return B[(start + end)/];
 }

 //获取中位数的中位数的下标
 int GetMidMid(int A[], int AIndex[], int k, int n)
 {
     if (k == n)
     {
         return AIndex[n];
     }

     int len_s = n - k + ;
     //筛选出n/5份的中位数
     int mod = len_s % ;
     int len = len_s /  + (mod != );
     for (int i = , j = k; i<= len && j <= n-mod; ++i, j+=)
     {
         B[i] = InsertSort(A, AIndex,j, j+);
     }
     if (mod != )
     {
         B[len] = InsertSort(A, AIndex, n - mod + , n);
     }
     return GetMidMid(A, B, , len);
 }

 //原址排序
 int Partition(int A[], int p, int n)
 {
     int pivot = A[n];
     int j = p - ;
     for (int i = p; i <= n - ; ++i)
     {
         if (A[i] <= pivot)
         {
             j++;
             swap(A[j], A[i]);
         }
     }

     swap(A[j + ], A[n]);
     return j + ;
 }

 int Select(int A[], int k, int n, int i)
 {
     if (k == n)
     {
         return A[n];
     }

     int midValueIndex = GetMidMid(A, AIndex, k, n);

     //将该中位数作为主元(pivot element)
     //使用一次原址重排
     int pivot = A[midValueIndex];
     swap(A[midValueIndex], A[n]);
     int mid = Partition(A, k, n);

     int t = mid - k + ;
     if (i == t)
     {
         return A[mid];
     }
     else if (i < t)
     {
         return Select(A, k, mid-, i);
     }
     else
     {
         return Select(A, mid+, n, i-t);
     }
 }
 int main(int argc, char** argv)
 {
     int n = ;
     for (int i = ; i <= n; ++i)
     {
         A[i] = i;
         AIndex[i] = i;
     }

     //for (int i = 1; i <= n; ++i)
     //{
     //    cout << A[i] << " ";
     //}
     //cout << endl;

     int equalNum = ;
     for (int i = ; i <= n; ++i)
     {
         //随机排列A数组
         for (int i = ; i <= n; ++i)
         {
             int j = i + rand() % nMax;
             //swap(A[i], A[j]);
             A[i] = j;
         }

         int ans1 = Select(A, , n, i);
         sort(A + , A + n + );
         int ans2 = A[i];

         if (ans1 == ans2)
         {
             equalNum++;
         }
     }
     cout << n << " " << equalNum << endl;
     return ;
 }