求第i小的元素

伪代码：

randomized_select(a,p,r,i)if p=r; then return a[p];q<-randomized_partition(a,p,r)k <- q-p+1if i=k then return a[q]else if i<k then return randomized_select(a,p,q-1,i) else return randomized_select(a,q+1,r,i-k)