Max Points on a Line

Approach 1: n^2 approach.
Calculate the slope of each pair of points, and count the max.
Be careful:
1. There can be same points in the points list.
2. Deal with 0, 1, 2 points in the list.
3. When (max > points.length/2) already found the max.
4. There can be points having same X or Y. If same Y, no slope.