Compilare: main.cpp: In function 'int main()': main.cpp:30:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < nn; i++) { ~~^~~~ main.cpp:34:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<str.length();i++) sol.push_back(str[i]); ~^~~~~~~~~~~~~ main.cpp:36:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < nn + n - 1; i++) out << sol[l - i - 1]; ~~^~~~~~~~~~~~