246.Strobogrammatic Number

    /*
     * 246.Strobogrammatic Number
     * 2016-6-18 by Mingyang
     * 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.
     * 我开始做没想到用hashmap，这么会使代码非常简单
     */
      public boolean isStrobogrammatic(String num) {
            HashMap<Character, Character> map = new HashMap<Character, Character>();
            map.put('1','1');
            map.put('0','0');
            map.put('6','9');
            map.put('9','6');
            map.put('8','8');
            int left = 0, right = num.length() - 1;
            while(left <= right){
                // 如果字母不存在映射或映射不对，则返回假
                if(!map.containsKey(num.charAt(right)) || num.charAt(left) != map.get(num.charAt(right))){
                    return false;
                }
                left++;
                right--;
            }
            return true;
        }