问题在这：
```cpp
for(int i=in.length()-1;i>=0;i--)
```
```cpp
i>=0
```
当它是负数时，in[0]='-'啊
建议特判
by AfterFullStop @ 2022-07-23 18:23:29
@[luogufather](/user/669020)
by AfterFullStop @ 2022-07-23 18:23:42
加一个特判，当是负数&&i==0时不输出
by detor @ 2022-07-24 22:33:22
```cpp
#include<bits/stdc++.h>
using namespace std;
int n,sum;
int main(int argc, char** argv) {
scanf("%d",&n);
if(n<0){//如果n为负数
cout<<"-";//那么输出负号
n=-n;
}
while(n){
sum=sum*10+n%10;//数的个位相当于新数的最高位(除十取余法)
n/=10;
}
cout<<sum<<endl;
return 0;
}
```
by lixuhui20120805 @ 2022-08-16 13:22:55
@[luogufather](/user/669020)
by lixuhui20120805 @ 2022-08-16 13:23:25
@[lixuhui20120805](/user/579795)
xiexei
by luogufather @ 2022-08-16 17:24:05
@[luogufather](/user/669020)
甭客气
by lixuhui20120805 @ 2022-08-16 17:35:30