change pivot in my quickSort algorithm java

Question

I have implemented a working quickSort algorithm using the first element in the array as the pivot, that look like this:

public int[] quickSort( int[] a, int st         


        

Answer 1

If you choose to start with pivoting on an arbitrary element you have to change the behavior of your partitioning loop. See the code below:

/* Selecting the pivot to be a random-ish element
   and pivotIndex to be beginning, since we don't know
   where it will be until we loop through the list */
int pivot = a[someInt];
int pivotIndex = begin-1;
//have to keep track of where the pivot actually is in the list
int currentPivotIndex = someInt;

for(int i = begin; i <= end; i++) {
    if(a[i] <= pivot) {
        //for each element less than the pivot
        //the pivotIndex moves by one
        pivotIndex++;
        //place the element to the left of the pivot
        this.swap(a, pivotIndex, i);
        //update currentPivotIndex if needed
        if(a[pivotIndex] == pivot) {
            currentPivotIndex = pivotIndex;
        }
    }
}
//put the pivot in its place
this.swap(a, pivotIndex, currentPivotIndex);