Quick sort's complexity varies greatly with the selection of pivot value. for example if you always choose first element as an pivot, algorithm's complexity becomes as worst as O(n^2). here is an smart method to choose pivot element- 1. choose the first, mid, last element of the array. 2. compare these three numbers and find the number which is greater than one and smaller than other i.e. median. 3. make this element as pivot element.
choosing the pivot by this method splits the array in nearly two half and hence the complexity reduces to O(nlog(n)).