Alternatively, a more careful deterministic pivot strategy can be used, such as median of medians. In practice this can be avoided by choosing a random element as pivot, which yields almost certain linear performance. This occurs for instance by taking the first element as the pivot and searching for the maximum element, if the data is already sorted. As with Quicksort, this has optimal average performance, in this case linear, but poor worst-case performance, in this case quadratic. Quickselect is a variant of quicksort – in both one chooses a pivot and then partitions the data by it, but while Quicksort recurses on both sides of the partition, Quickselect only recurses on one side, namely the side on which the desired kth element is. Linear performance can be achieved by a partition-based selection algorithm, most basically quickselect.
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