246. Strobogrammatic Number

O(n) time O(1) space

class Solution {
public:
    bool isStrobogrammatic(string num) {
        int m[] = {0, 1, -1, -1, -1, -1, 9, -1, 8, 6};
        for (int n = num.length(), l = 0, r = n - 1; l <= r; ++l, --r) {
            int cl = num[l] - '0', cr = num[r] - '0';
            if (m[cl] == -1 || m[cr] == -1 || m[cl] != cr) return false;
        }
        return true;
    }
};