the set of points in the convex hull
List < List < int > > a = new List < List < int > > ( ) ; a . Add ( new List < int > ( ) { 0 , 0 } ) ; a . Add ( new List < int > ( ) { 3 , - 1 } ) ; a . Add ( new List < int > ( ) { 4 , 5 } ) ; a . Add ( new List < int > ( ) { - 1 , 4 } ) ; int n = a . Count ; List < int > p = new List < int > ( ) { 100 , 100 } ; addPoint ( a , p ) ;