Straight Digital Numbers

In this challenge, you have to establish if the digits of a given number form a straight arithmetic sequence (either increasing or decreasing). A straight sequence has an equal step between every pair of digits.

Given an integer n, implement a function that returns:

• "Not Straight" if n is lower than 100 or if its digits are not an arithmetic sequence.
• "Trivial Straight" if n has a single repeating digit.
• An integer being the step of the sequence if the n digits are a straight arithmetic sequence.

Examples

straightDigital(123) ➞ 1
// 2 - 1 = 1 | 3 - 2 = 1

straightDigital(753) ➞ -2
// 5 - 7 = -2 | 3 - 5 = -2

straightDigital(666) ➞ "Trivial Straight"
// There's a single repeating digit (step = 0).

straightDigital(124) ➞ "Not Straight"
// 2 - 1 = 1 | 4 - 2 = 2
// A valid sequence has always the same step between its digits.

straightDigital(99) ➞ "Not Straight"
// The number is lower than 100.

Notes

• The step of the sequence can be either positive or negative (see example #2).
• Trivia: there are infinite straight digital numbers, but only 96 of them are made of at least two different digits.