Compilare: user.cpp: In function ‘int main()’: user.cpp:27:49: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] { s1[a][b]=NULL; ^ user.cpp:41:67: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] { for(b=0;b<=l[a];b++) ^ user.cpp:43:67: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(b=0;b<=l[u];b++) ^ user.cpp:45:67: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(b=0;b<=l[a];b++) ^ user.cpp:85:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(j=0;j<l[i];j++) ^