Compilare: main.cpp: In function 'int main()': main.cpp:30:20: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] s1[i][n+1]=NULL; ^~~~ main.cpp:5:32: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("nks.in","r",stdin); ^ main.cpp:6:34: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("nks.out","w",stdout); ^ main.cpp:8:39: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result] scanf("%d%d%d%d%d",&m,&n,&p,&q,&s); ^