why does the app not accept my code, if it is correct

Question

#include <iostream>
#include <string.h>
using namespace std;

int main() {

string cadena;
string new_str;
cout<<"Introduzca la palabra: ";
getline (cin, cadena );
for(int i = cadena.length();i>=0;i--){
new_str += cadena[i];
}
cout<<new_str<<endl;

return 0;
}

Answer 1

https://code.sololearn.com/cmn3Dj64TGJE/?ref=app
A bit rectification is required compare your code with this