convexHull

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given a set of points, find the convex hull (smallest convex set containing all the points)
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brute-force algorithm:
(1)based on the two observation: edges of convex hull of P connect pairs of points in P
(2)p-q is on convex hull if all other points are CCW of pq.
we have: 
for all pairs of points p and q in P, compute ccw(p,q,x) for all other x in P,  p-q is on hull if all values are positive


package wrap algorithm:
start with point with smallest y coord, rotate sweep line around current point in ccw direction,
first point hit is on the hull, we update current point and repeat the process.


Graham Scan algorithm:
choose point p with smallest y-coord.
sort remaining points by polar angle w.r.t p to get simple polygon.
we consider points in simple polygon in order, and discard those that would create a clockwise turn.