The main idea is that we don't want to generate all numbers. it would be slow and waste many space. It's easy to implement a function that calculate the number of strobogrammatic numbers with k digits.
Then we want to calculate that if there is a lower bound, how many strobo numbers with k digits could we have?
That could be solved by using D&C. The logic is in the function strobogrammaticNotLessThanKWithSameDigits.