Selection

-      find the k-th smallest key in a list

** quickselect(index j)

http://m.blog.naver.com/babobigi/220527978173

-      modification of quicksort

-      θ(n) average-case time complexity

1.     start with an unsorted list I of n input items

2.     choose a pivot item v from I

3.     Partition I into unsorted lists I1, I2 and Iv like quicksort

4.     If (j<=|I1|){

Recursively find the item with index j in I1;

Return it;

}else if(j<=|I1|+|Iv|){

      Return the pivot v;

}else{

      Recursively find the item with index j-|I1|-|Iv| in I2;

      Return it;    

}