线性时间选择

template<class Type>
Type select (Type a[],int p, int r, int k)
   {
      if (r-p<75) {
        //用某个简单排序算法对数组a[p:r]排序;
        bubbleSort(a,p,r);
        return a[p+k-1];
        }
      //将a[p+5*i]至a[p+5*i+4]的第3小元素
      //与a[p+i]交换位置;
      //找中位数的中位数，r-p-4即上面所说的n-5
      for ( int i = 0; i<=(r-p-4)/5; i++ )
      {
         int s=p+5*i,
             t=s+4;
         for (int j=0;j<3;j++) bubbleSort(a,s,t-j);
         swap(a, p+i, s+2);
      }
      Type x = select(a,p, p+(r-p-4)/5, (r-p+6)/10);
      int i=partition(a,p,r,x),j=i-p+1;
      if (k<=j) return select(a,p,i,k);
      else return select(a,i+1,r,k-j);
   }

#include <iostream>

using namespace std;

template<class Type>
void swap(Type a[],int p,int r){
    Type temp=a[p];
    a[p]=a[r];
    a[r]=temp;
}

template<class Type>
void bubbleSort(Type a[],int p,int r){
    int lastSwapPos = 0,lastSwapPosTemp = 0;
    for (int i = p; i < r; i++)  {  
        lastSwapPos = lastSwapPosTemp;
        for (int j = r; j >lastSwapPos; j--)  {  
            if (a[j - 1] > a[j]){  
                swap(a,j-1,j);
  
                lastSwapPosTemp = j;  
            }  
        }
        
        if (lastSwapPos == lastSwapPosTemp)  
            break;  
    }
}

template <class T>
int partition (T a[], const int p, const int r,T x) {
    int pivotpos = p;
    for (int i = p; i <= r; i++)
    if (a[i] < x) {
        pivotpos++;
        if (pivotpos != i){
            swap(a,pivotpos,i);
        }
    }
    return pivotpos;
}


template<class Type>
Type select (Type a[],int p, int r, int k){
    if (r-p<75) {
    //对数组a[p:r]排序;
    bubbleSort(a,p,r);
    return a[p+k-1];
    }
    
    //将a[p+5*i]至a[p+5*i+4]的第3小元素
    //与a[p+i]交换位置;
    //找中位数的中位数，r-p-4即上面所说的n-5
    for ( int i = 0; i<=(r-p-4)/5; i++ ){
     int s=p+5*i,
         t=s+4;
     for (int j=0;j<3;j++) bubbleSort(a,s,t-j);
     swap(a, p+i, s+2);
    }
    
    Type x = select(a,p, p+(r-p-4)/5, (r-p+6)/10);
    int i=partition(a,p,r,x),j=i-p+1;
    if (k<=j) return select(a,p,i,k);
    else return select(a,i+1,r,k-j);
}


int main() {
    
    //int a[]={0,6,5,1};
    //int a[]={1,2,3,4,5,6,7,8,9,10};
    int a[]={1,29,3,4,5,9,7,855,97,100};
    cout<<select(a,0,9,10);

    return 0;
}