/in/foo.cc: In function 'int main()':
/in/foo.cc:11:6: error: 'i' was not declared in this scope
  for(i=0;i<n;i++)
      ^
/in/foo.cc:13:2: error: 'i' was not declared in this scope
  i=0;
  ^
/in/foo.cc:14:28: warning: NULL used in arithmetic [-Wpointer-arith]
  while(a[i]!=NULL || b[k]!=NULL)
                            ^~~~
/in/foo.cc:19:5: error: lvalue required as increment operand
    c++;
     ^~
/in/foo.cc:25:5: error: lvalue required as increment operand
    c++;
     ^~
/in/foo.cc:31:15: warning: NULL used in arithmetic [-Wpointer-arith]
   while(b[k]!=NULL)
               ^~~~
/in/foo.cc:34:5: error: lvalue required as increment operand
    c++;
     ^~
/in/foo.cc:38:11: warning: NULL used in arithmetic [-Wpointer-arith]
  if(b[k]==NULL)
           ^~~~
/in/foo.cc:43:5: error: lvalue required as increment operand
    c++;
     ^~