246. Strobogrammatic Number

题目：

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.

链接： http://leetcode.com/problems/strobogrammatic-number/

3/2/2017

performance不好，至少可以不用hashset而用hashmap，这样6，9就可以跟其他字母一样对待了。

public class Solution {
    public boolean isStrobogrammatic(String num) {
        HashSet<Character> s = new HashSet<Character>(Arrays.asList('1', '8', '0'));
        for (int i=0, j=num.length() - 1; i<=j; i++, j--) {
            char a = num.charAt(i);
            char b = num.charAt(j);
            if (a == b && s.contains(a) || a == '6' && b == '9' || a == '9' && b == '6') continue;
            return false;
        }
        return true;
    }
}