Given a string num which represents an integer, return true if num is a strobogrammatic number. A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down). 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 Constraints: 1 <= num.length <= 50 num consists of only digits. num does not contain any leading zeros except for zero itself.