Quicksort

qsort:{:[#x;(qsort x@&x[0]>/:x),x[0],(qsort x@&x[0]</:x);()]}
qsort:{:[#x;(qsort x@&x[0]>/:x),(x@&x[0]=/:x),(qsort x@&x[0]</:x);()]}

The former variant removes duplicates while the latter preserves them.