Compilare: user.cpp: In function ‘int main()’: user.cpp:25:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for ( long i=0;i<a.size();++i) ^ user.cpp:44:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (long i = 0;i<a.size();++i) ^ user.cpp:15:37: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("alibaba.in","r",stdin) ; ^ user.cpp:16:39: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("alibaba.out","w",stdout) ; ^ user.cpp:17:31: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf ("%ld %ld\n",&n,&k) ; ^ user.cpp:18:15: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(b+1) ; ^ /tmp/cc1hM5Aj.o: In function `main': user.cpp:(.text.startup+0x72): warning: the `gets' function is dangerous and should not be used.