Solution.cpp

class Solution {
public:
    string nearestPalindromic(string n) {
        long x = stol(n);
        long ans = -1;
        for (long t : get(n))
            if (ans == -1 || abs(t - x) < abs(ans - x) || (abs(t - x) == abs(ans - x) && t < ans))
                ans = t;
        return to_string(ans);
    }

    unordered_set<long> get(string& n) {
        int l = n.size();
        unordered_set<long> res;
        res.insert((long) pow(10, l - 1) - 1);
        res.insert((long) pow(10, l) + 1);
        long left = stol(n.substr(0, (l + 1) / 2));
        for (long i = left - 1; i <= left + 1; ++i) {
            string prefix = to_string(i);
            string t = prefix + string(prefix.rbegin() + (l & 1), prefix.rend());
            res.insert(stol(t));
        }
        res.erase(stol(n));
        return res;
    }
};