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.

思路： 两边到内check是否为 00 11 88 69 96
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

public boolean isStrobogrammatic(String num) {
    for (int i=0, j=num.length()-1; i <= j; i++, j--)
        if (!"00 11 88 696".contains(num.charAt(i) + "" + num.charAt(j)))
            return false;
    return true;
}