2384. Largest Palindromic Number

You are given a string num consisting of digits only.

Return the largest palindromic integer (in the form of a string) that can be formed using digits taken from num. It should not contain leading zeroes.

Notes:

• You do not need to use all the digits of num, but you must use at least one digit.
• The digits can be reordered.

c++

class Solution {
public:
    string largestPalindromic(string num) {
        unordered_map<char, int> freq;
        for(auto& n : num) freq[n]++;
        string res = "";
        char ma = '#';
        for(char ch = '9'; ch >= '0'; ch--) {
            int cnt = freq[ch] / 2;
            res += string(cnt, ch);
            freq[ch] %= 2;
        }
        for(char ch = '0'; ch <= '9'; ch++) {
            if(freq[ch]) ma = ch;
        }
        if(res[0] == '0') {
            if(ma == '#') return "0";
            return string(1,ma);
        }
        string rev = res;
        reverse(begin(rev), end(rev));
        if(ma != '#') return res + ma + rev;
        return res + rev;
    }
};