Compilare: user.cpp: In function ‘void pop()’: user.cpp:16:19: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] s[strlen(s)-1]=NULL; ^ user.cpp: In function ‘int main()’: user.cpp:31:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=1;i<=strlen(a)-1;i++) ^ user.cpp:36:11: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wparentheses] if (s[n]!=a[i]) ^