Given a string num representing an integer, determine whether it is a strobogrammatic number. Return TRUE if the number is strobogrammatic or FALSE if it is not.

Note: A strobogrammatic number appears the same when rotated $180$ degrees (viewed upside down). For example, “69” is strobogrammatic because it looks the same when flipped upside down, while “962” is not.

Constraints:

• $1 <=$ num.length $<= 50$

• num contains only digits.

• num has no leading zeros except when the number itself is zero.

The solution uses a two pointer approach to determine whether a given string num is a strobogrammatic number by checking its digits from both ends toward the center. It uses a set of valid digit mappings that remain unchanged when rotated $180$ degrees or transform into each other when flipped (such as $'0'$ ...