246. Strobogrammatic Number

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<Character, Character> dict = new HashMap<>();
        dict.put('0','0');
        dict.put('1','1');
        dict.put('8','8');
        dict.put('6','9');
        dict.put('9','6');
        int i=0; 
        int j = num.length()-1;
        while(i<=j){
            char f = num.charAt(i);
            char b = num.charAt(j);
            if(dict.containsKey(f) && dict.containsKey(b) && dict.get(f) == b){
                i++;j--;
            }else{
                return false;
            }
        }
        return true;

    }
}