PRINT DISTINCT PALINDROMIC PERMUTATIONS OF A STRING

/*
link- https://www.lintcode.com/problem/917/description

Given a string s, return all palindromic permutations ( without repetitions ). Returns an empty list if there are no palindromic permutations.
sample 1
S = "aabb"  ANSWER-> ["abba","baab"]
sample 2
S = "abc"  ANSWER-> []
*/


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class Solution {
public:
    void generate(string str, int curr, vector<string> &res){
        if(str.size()==curr){
            res.push_back(str);
            return;
        }
        for(int i=curr;i<str.size();i++){
            if(i!=curr && str[i]==str[curr]){
                continue;
            }
            swap(str[i],str[curr]);
            generate(str,curr+1,res);
        }
    }

    vector<string> generatePalindromes(string &s) {
        int freq[26]={0};
        for(auto x: s){
            freq[x-'a']++;
        }
        int odd=0;
        char midChar='#';
        for(int i=0;i<26;i++){
            if(freq[i]%2!=0){
                midChar=(i+'a');
                odd++;
            }
        }
        if(odd>1){
            return {};
        }
        string str="";
        for(int i=0;i<26;i++){
            int f=freq[i]/2;
            while(f--){
                str+=(i+'a');
            }
        }
        vector<string> res;
        generate(str,0,res);
        for(int i=0;i<res.size();i++){
            string curr=res[i];
            string rev=curr;
            reverse(rev.begin(),rev.end());
            if(midChar!='#'){
                curr+=midChar;
            }
            res[i]=curr+rev;
        }
        return res;
    }
};