A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).
Write a function to count the total strobogrammatic numbers that exist in the range of low <= num <= high.
Given low = "50", high = "100", return 3. Because 69, 88, and 96 are three strobogrammatic numbers.
Because the range might be a large number, the low and high numbers are represented as string.