修复了进入倒计时时原先时间保留的问题

labcodes_answer/lab1_result/kern/trap/trap.c

@@ -225,6 +225,7 @@ trap_dispatch(struct trapframe *tf) {
             mode = COUNTDOWN;
             cprintf(MSG_COUNTDOWN_START);
             state = WAITING_FOR_INPUT;
+            milliseconds = 0;
             break;
         case 's':
         case 'c':