Compilare: user.cpp: In function ‘void difference(int, int)’: user.cpp:71:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(j < (int)stk[b].size()-1 && stk[b][j] < stk[a][i]) j++; ^ user.cpp: In function ‘int main()’: user.cpp:190:14: warning: NULL used in arithmetic [-Wpointer-arith] if(*p != NULL) ^ user.cpp:184:35: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("expr.in", "r", stdin); ^ user.cpp:185:37: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("expr.out", "w", stdout); ^ user.cpp:187:12: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(s); ^ /tmp/ccUZIi76.o: In function `main': user.cpp:(.text.startup+0x54): warning: the `gets' function is dangerous and should not be used.