Division and Recursion-find middle element

Posted 2020-06-30

#include <iostream>
using namespace std;

void swap(int array[], int beginPos, int endPos){
    int t = array[beginPos];
    array[beginPos] = array[endPos];
    array[endPos]=t;
}

int partition(int array[], int begin, int end, int val)
{
    int headCursor = begin;
    int tailCursor = end;

    while(true)
    {
        while(array[headCursor] < val && headCursor < end) headCursor++;
        while(array[tailCursor] > val) tailCursor--;
        if (headCursor >= tailCursor){break;}
        swap(array, headCursor, tailCursor);
    }

    return tailCursor;
}

int partition2(int array[], int beginPos, int endPos)
{
    int comparedVal = array[beginPos];
    int cursorFromBegin = beginPos + 1;
    int cursorFromEnd = endPos;

     while(true){
         while (array[cursorFromBegin] < comparedVal){cursorFromBegin++;}
         while (array[cursorFromEnd] > comparedVal){cursorFromEnd--;}
         if (cursorFromBegin>=cursorFromEnd){break;}
         swap(array, cursorFromBegin, cursorFromEnd);
     }
     swap(array,cursorFromEnd,beginPos);
     return cursorFromEnd;
}

void quickSort(int array[],int begin,int end){
    if (begin  >= end){cout<<begin<<":"<<end<<endl;return;}
    int k =  partition2(array,begin,end);
    quickSort(array, begin,k-1);
    quickSort(array, k+1, end);
}

int select_middle_element(int array[], int begin,int end, int k){
    if (end - begin <=10){
        quickSort(array, begin,end);
        return array[(end+begin)/2];
    }

    int groupNum = (end-begin-4)/5;
    int groupBegin = begin;
    int groupEnd = begin + 4;
    int arraybegin = begin;
   
    for (int i = 0; i <groupNum;i++){
        quickSort(array, groupBegin, groupEnd);
        swap(array, arraybegin, (groupBegin+groupEnd)/2);
        groupBegin = groupEnd + 1;
        groupEnd = groupBegin +4;
        arraybegin++;
    }
   
    int middleVal = select_middle_element(array, begin, begin+(groupNum-1), (groupNum/2));

    

    cout<<"middle "<<middleVal<<endl;
    int pos = partition(array, begin, end, middleVal);
    int j = pos - begin;

    cout<<"pos "<<pos<<endl;
    cout<<"k "<<k<<endl;
    cout<<"begin"<<begin<<endl;

     for (int i = 0;i<=24;i++)
     {
         cout<<array[i]<<" ";
     }
     cout<<endl;

     for (int i = begin;i<=end;i++)
     {
         cout<<array[i]<<" ";
     }
     cout<<endl;
    if (k > j){
        return select_middle_element(array, pos, end, k-j);
    }else if (k==j){return array[k+begin];}
    else{
        return select_middle_element(array, begin, pos, k);
    }

    return k;
}

int main(){
    int array[25]={4,7,1,2,3,
                   9,8,10,13,56,
                   90,100,877,900,1000,
                   30,89,40,45,28,
                   85};
   
    cout<<"middle value"<<select_middle_element(array, 0, 20, 10)<<endl;
    quickSort(array, 0, 20);

    /*1 if n <=75， get the middle element .*/
    /*1 divide the n elements into n-4/5 gorups, find the middle element of each group
    (every group only have 5 elements), and find the middle element among the middle elements
    of each group, and then kick out those elements that are not possible*/
    for (int i = 0;i<=20;i++)
    {
        cout<<i<<":"<<array[i]<<endl;
    }
}