多问多练，当遇到不懂的，看到答案却看懂了，这算我懂了还是不懂........
int num = 999;
int count = 0;
if (num >= 0 && num<=999999999){
while(num != 0){
count++;
num/=10;
}
System.out.println("它是个"+ count+"位的数！");
} else{
System.out.println("输入有误！");
