next greater palindrome, C++ (gcc)

next greater palindrome

#include <bits/stdc++.h>

using namespace std;

int main(){
    ios_base :: sync_with_stdio(0);
    cin.tie(0);
    
    int t;
    cin >> t;
    while(t--){
        string s;
        cin >> s;
        int nine = 0;
        for(int j = 0; j < s.length(); j++) if(s[j] == '9') nine++;
        if(nine == s.length()){
            cout << 1;
            for(int j = 0; j < s.length()-1; j++) cout << 0;
            cout << 1 << endl;
        }
        else{
            string ans = s;
            int n = s.length();
            for(int j = n/2; j < n; j++)
                ans[j] = ans[n - j - 1];
            if(ans > s) cout << ans << endl;
            else{
                if(n % 2 != 0 && ans[n/2] != '9') ans[n/2]++, cout << ans << endl;
                else{
                    if(n%2) ans[n/2] = '0';
                    int l = n/2 - 1, r = l+1 + (n%2);
                    while(l >= 0){
                        if(ans[l] == '9'){
                            ans[l] = ans[r] = '0';
                            l--;
                            r++;
                            continue;
                        }
                        ans[l]++, ans[r]++;
                        break;
                    }
                    cout << ans << endl;
                }
            }
        }
    }
    return 0;
}


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