# 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.

**The Idea:** Good reference: [https://oeis.org/A000787](https://oeis.org/A000787). A number is strobogrammatic if it is pair wise palindromic. That is, looking at the vertical reflection of the number, 8 maps with 8, 9 maps with 6, 1 maps with 1, ... etc.

**Complexity:** O(n) time and O(1) space

```python
class Solution:
    def isStrobogrammatic(self, num):
        """
        :type num: str
        :rtype: bool
        """
        if not num:
            return False

        sym_pair = {'8': '8', '6': '9', '9': '6', '1': '1', '0': '0'}
        left = 0
        right = len(num) - 1

        while left <= right:
            l_val = num[left]
            r_val = num[right]
            if l_val not in sym_pair or r_val not in sym_pair:
                return False
            elif sym_pair[l_val] != r_val or sym_pair[r_val] != l_val:
                return False
            left += 1
            right -= 1

        return True


obj = Solution()
print(obj.isStrobogrammatic('69'))
for sn in [str(num) for num in [0, 1, 8, 11, 69, 88, 96, 101, 111, 181, 609, 619, 689, 808, 818, 23, 9199]]:
    print(obj.isStrobogrammatic(sn))
```
