如何计算线性时间的排列，扭曲

I have a resource scheduling issue in Java where things need to be sequenced, but there are restrictions on what resources can be next to each other. A good analogy is a string of "digits", where only certain digits can be next to each other. My solution was recursive, and works fine for small strings, but run time is O(X^N), where X is the number of possible digits (the base), and N is the length of the string. It quickly becomes unmanageable.I have a resource scheduling issue in Java wher