Strobogrammatic Number leetcode

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.

For example, the numbers "69", "88", and "818" are all strobogrammatic.

public class Solution {
    public boolean isStrobogrammatic(String num) {
        HashMap map = new HashMap();
        map.put('1', '1');
        map.put('0', '0');
        map.put('6', '9');
        map.put('8', '8');
        map.put('9', '6');
        int left = 0;
        int right = num.length() - 1;
        while (left <= right) {
            if (!map.containsKey(num.charAt(left)) || map.get(num.charAt(left)) != num.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}