Compilare: main.cpp: In function 'int main()': main.cpp:36:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=1;i<a.size();i++) ~^~~~~~~~~ main.cpp:23:33: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("prefix.in","r",stdin); ^ main.cpp:24:35: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("prefix.out","w",stdout); ^