代码如下：
 #include<stdio.h>

main()
{
   int num,fannum,i,n,m;
   printf("This program reverses the digits in an interger.\n");
   printf("Enter a positive integer:");
   scanf("%d",&num);
   n=num;
   i=0;
   m=1;
   while (n>0) {
       n/=10;
       i++;
       m*=10;
   }
   fannum=0;
   m/=10;
   while (num>0) {
       n=num%10;
       fannum+=n*m;
       m/=10;
       num/=10;
       i--;
   }
   printf("The reversed number is %d\n",fannum);
}
在TC2.0下运行，输入5位数就会输出负数，搞不明白啊。

#include<stdio.h>

main()
{
   long num,fannum,i,n,m;
   printf("This program reverses the digits in an interger.\n");
   printf("Enter a positive integer:");
   scanf("%ld",&num);
   n=num;
   i=0;
   m=1;
   while (n>0) {
       n/=10;
       i++;
       m*=10;
   }
   fannum=0;
   m/=10;
   while (num>0) {
       n=num%10;
       fannum+=n*m;
       m/=10;
       num/=10;
       i--;
   }
   printf("The reversed number is %ld\n",fannum);
}

谢谢，变量的类型弄错了