Compilare: main.cpp: In function 'int main()': main.cpp:83:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (used.size()!=n){ ~~~~~~~~~~~^~~ main.cpp:40:30: warning: ignoring return value of 'size_t fread(void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Wunused-result] fread (buff,1,400000,fin); ^