如何更改我的快速排序代码以使用数组的最后一个元素作为 pivot？

Question

Can someone explain me how i can change the following code to use the last element of my array as the pivot? In this code example i am using the first element as my pivot.

I found a couple of other codes but they are sorting the array in a different way which is not what i want.

In my attempts to change the pivot to use the last element i didn't get an errorcode but the sorting failed in the middle of my array. Any help is very much appreciated.

int partition(vector<int>& v,int low, int high){
    int pivot = v[low];
    int ui{low};
    int oi{high + 1};
    
    while(true){

        while(v[++ui] < pivot){
            if(ui == high){
                break;
            }
        }
        
        while(pivot<v[--oi]){
            if(oi == low){
                break;
            }
        }
        
        if(ui >= oi){
            break;
        }
        swap(v[ui],v[oi]);
    }
    
    swap(v[low],v[oi]);
    
    return oi;
}

void quicksort(vector<int>& v, int low, int high){
    
    if(high>low){
        
        int pivot = partition(v,low,high);
        
        quicksort(v,low, pivot - 1);
        quicksort(v,pivot + 1,high);
    }
}

Answer 1

It does not matter where you choose your pivot. All that matters is that you chose one. You could even choose a random one each time.

Once your pivot is chosen, then you can start moving elements around, using the place where the pivot was as the initial unused, move-to spot when shuffling elements.

When you are done with the partitioning, your unused, move-to spot should be between the left and right sides. Stick the pivot value there, and return the index of that spot to the caller.

Answer 2

To switch from using low as the pivot index to using high , you need to following changes:

int partition(vector<int>& v,int low, int high){
    // before:
    //int pivot = v[low];
    //int ui{low};
    //int oi{high + 1};

    // after:
    int pivot = v[high];
    int ui{low - 1};
    int oi{high};

    while(true){

        while(v[++ui] < pivot){
            if(ui == high){
                break;
            }
        }
        
        while(pivot<v[--oi]){
            if(oi == low){
                break;
            }
        }
        
        if(ui >= oi){
            break;
        }
        swap(v[ui],v[oi]);
    }
    
    //before:
    //swap(v[low],v[oi]);
    
    //after:
    swap(v[high],v[oi]);
    
    return oi;
}