Special method for splitting array in two = PARTITION
Pick an avergare data value = PIVOT
and arrange the data so that everything < pivot is to its left
and everything >= pivot is to its right.

DATA =      7  5  2  4  8  1  3  6  9  2  5  4, pick pivot = 5
partition around the five
            2  4  1  3  2  1  3  9  6  8  5  7
            \______less______/   \____>=_____/
                sort this          sort this
            1  1  2  2  3  3  4  5  6  7  8  9
merge is not required.