[lintcode] problem 644 – strobogrammatic number

public boolean (String num) {
    Map<Character, Character> map = new HashMap<Character, Character>(){{
        put('0', '0');
        put('1', '1');
        put('6', '9');
        put('8', '8');
        put('9', '6');
    }};

    int i = 0;
    int j = num.length() - 1;

    while (i <= j) {
        if (!map.containsKey(num.charAt(i)) || map.get(num.charAt(i)) != num.charAt(j))
            return false;

        i++;
        j--;
    }

    return true;
}