foo.cpp: In function 'int main()':
foo.cpp:9:25: error: 'memset' was not declared in this scope
   memset(a, 0, sizeof(a));
                         ^
foo.cpp:7:6: warning: unused variable 'num' [-Wunused-variable]
  int num, a[2501], b[2501], c[5002];
      ^