Firstly I really wouldn't recommend using global variables.
But in the interests of a short answer to the point of the question and using your chosen method for sharing data (this same idea would work equally using a better method of data sharing between buttons):
Just create a second (global) variable called lastLL (or preferably choose a more meaningful variable name than 'LL', but again, that is a side-issue!) and before each of your buttons applies their technique to overwrite LL just set
lastLL = LL;
Then in your undo button just set
LL = lastLL;
