Finding the convex hull

The convex hull of a set Q of points is the smallest convex polygon for which each point in Q is either on the boundary or inside the polygon. We discuss two techniques to solve these called Graham's scan and Jarvis' march.
Graham's approach is based on the following steps:
1) Choose the point with the lowest y-coordinate and the left most in case of a tie as the starting point.
2) Push this point and the next two points visited in the counter clockwise order on stack S
3) For the next points if the angle formed by the next to top and the top and the candidate point makes a non-left turn, then pop it from the stack
otherwise push the next point on the stack and proceed
The stack returns the convex hull vertices.
The complexity is O(nlogn)
Jarvis' approach also known as package wrapping is based on the following steps:
We start with the lowest point and we go around the board building a sequence such that the next vertex in the convex hull has the smallest polar angle with respect to the previous point and in case of ties we pick the point farthest from the previous point. When we reach the highest vertex, breaking ties by choosing the farthest such vertex.
The complexity is O(NM)