Compilare: user.cpp: In function ‘int main()’: user.cpp:23:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 1; i < strlen(S); i++) ^ user.cpp:57:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (k < strlen(S) - 1) ^ user.cpp:12:40: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("charlie.in","r",stdin); ^ user.cpp:13:42: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("charlie.out","w",stdout); ^ user.cpp:15:26: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d\n", &N); ^ user.cpp:16:16: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(S); ^ /tmp/ccOIcgwQ.o: In function `main': user.cpp:(.text.startup+0x6a): warning: the `gets' function is dangerous and should not be used.