Compilare: user.cpp: In function ‘void rec(int, int&)’: user.cpp:28:51: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] for(v=k1;v<=k;v++) s2[v-k1]=s[v]; s2[v-k1]=NULL; ^ user.cpp: In function ‘int main()’: user.cpp:54:48: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] for(k=j;k<=i;k++) s2[k-j]=s[k]; s2[k-j]=NULL; ^ user.cpp:74:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(j=0;j<strlen(s1[i]);j++) ^