1842. Next Palindrome Using Same Digits

You are given a numeric string num, representing a very large palindrome.

Return the smallest palindrome larger than num that can be created by rearranging its digits. If no such palindrome exists, return an empty string “”.

A palindrome is a number that reads the same backward as forward.

class Solution {
public:
    string nextPalindrome(string num) {
        string half = num.substr(0, (num.length()) / 2);
        next_permutation(begin(half), end(half));
        
        string rev = half;
        reverse(begin(rev), end(rev));
        char mid = num.length() & 1 ? num[num.length() / 2] : '#';
        string res = mid != '#' ? half + mid + rev : half + rev;
        
        for(int i = 0; i < num.length(); i++) {
            if(num[i] < res[i]) return res;
            else if(num[i] == res[i]) continue;
            else return "";
        }
        
        return "";
    }
};