A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).
Write a function to determine if a number is strobogrammatic. The number is represented as a string.
Example 1:
Input: num = "69" Output: true
Example 2:
Input: num = "88" Output: true
Example 3:
Input: num = "962" Output: false
Example 4:
Input: num = "1" Output: true