Compilare: user.cpp: In function ‘void sol()’: user.cpp:28:81: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] if(G[i].empty()) {int v; v=s[nrs-1]-'0'; v++; f[i]=v; s[nrs]=v+'0'; s[nrs+1]=NULL; } ^ user.cpp:39:71: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] if(strchr(s,c)==NULL) {f[i]=p; nrs++; s[nrs-1]=c; s[nrs]=NULL;} ^ user.cpp:45:64: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] if(strchr(s,c)==NULL) {f[i]=p; nrs++; s[nrs-1]=c; s[nrs]=NULL;} ^