fix exercise 1-12 edge cases

Author: caluckenbach

chapter_1/exercise_1_12/copy_io_nl.c

@@ -2,22 +2,21 @@
 
 int main()
 {
-  char c;
-  char last_char = -1;
+  int c, prev;
+  prev = EOF;
 
   while ((c = getchar()) != EOF) {
-    if((c==' ' ||  c=='\t' ||  c=='\n')) {
-      if(c != last_char) {
+    if (c == ' ' || c == '\t' || c == '\n') {
+      if (prev != ' ' && prev != '\t' && prev != '\n') {
         putchar('\n');
       }
     }
     else {
       putchar(c);
     }
 
-    last_char=c;
+    prev = c;
   }
 
   return 0;
 }
-