[LeetCode] Strobogrammatic Number

Problem Description:

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.

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The following is the C++ implementation of the suggested solution using a look-up table (implemented as an unordered_map). It takes 0 ms. But, I wonder, are there any real applications of strobogrammatic numbers?

class Solution {
public:
    bool isStrobogrammatic(string num) {
        make_lut();
        int n = num.length(); 
        for (int l = 0, r = n - 1; l <= r; l++, r--)
            if (lut.find(num[l]) == lut.end() || lut[num[l]] != num[r])
                return false;
        return true;
    }
private:
    unordered_map<char, char> lut; 
    void make_lut(void) {
        lut['0'] = '0';
        lut['1'] = '1';
        lut['6'] = '9';
        lut['8'] = '8';
        lut['9'] = '6';
    }
};