Problem 661. Spot the outlier

All points except for one lie on a line. Which one is the outlier?

Example:

You are given a list of x-y pairs in a column like this:

 pts = [ 0 1 
         0 2 
         3 2
         0 3 
         0 4 ]

You would return the number 3, since the third point is the only one that is non-collinear with the other points. All the others are on the y-axis.

 outlier = 3