Problem 317. Find the stride of the longest skip sequence

We define a skip sequence as a regularly-spaced list of integers such as might be generated by MATLAB's colon operator. We will call the inter-element increment the stride. So the vector 2:3:17 or [2 5 8 11 14 17] is a six-element skip sequence with stride 3.

Given the vector a, your job is to find the stride associated with the longest skip sequence you can assemble using any of the elements of a in any order. You can assume that stride is positive and unique.

Example:

 input  a = [1 5 3 11 7 2 4 9]
 output stride is 2

since from the elements of a we can build the six-element sequence [1 3 5 7 9 11].