9.6.1 Algorithm:

To sort the records a[i], a[i + 1],..., a[j], in place.
quicksort (i, j)

    {

        if (a[i] ^... a[j] contain at least two distinct keys)

            {

                let v be the larger of the first two distinct keys;

                Partition a[i] ^... a[j] so that for some k between i + 1 and j,

a[i] ^... a[k - 1] all have keys <v, and

a[k] ^... a[j] all have keys $\geq$v;

                quicksort (i, k - 1);

                    quicksort (k, j);

        }

    }

$$\underbrace{a[i] \space \cdots \space a[k-1]}_{\mbox{keys} \space < v}^{}\,$$$$\underbrace{a[k] \space \cdots \space a[j]}_{\mbox{keys} \space \geq v}^{}\,$$ v is called the pivot. It could be any element such that the resulting partition desirably has two equal sized groups.