code cleanup

counter.cpp

@@ -7,11 +7,11 @@ Counter::Counter() {
 }
 
 void Counter::start() {
-    gettimeofday(&startTime, NULL);
+    gettimeofday(&startTime, nullptr);
 }
 
 long Counter::getTime() {
-    gettimeofday(&endTime, NULL);
+    gettimeofday(&endTime, nullptr);
     long back = (endTime.tv_usec - startTime.tv_usec) / 1000 +
                 (endTime.tv_sec - startTime.tv_sec) * 1000;
     this->start();