Compilare: user.cpp: In function ‘int simetric(int, int)’: user.cpp:45:9: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] s1[2*l]=NULL; s2[2*l]=NULL; ^ user.cpp:45:23: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] s1[2*l]=NULL; s2[2*l]=NULL; ^ user.cpp: In function ‘void back(int)’: user.cpp:52:5: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wparentheses] if (!simetric(level-1,n)) ^ user.cpp: In function ‘void load()’: user.cpp:18:26: warning: ignoring return value of ‘int fscanf(FILE*, const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] fscanf(fp,"%d %d",&n,&k); ^