关键是

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<math.h>

void print(int n)
{
   if (n > 9)
   {
      print(n / 10);
   }
   printf("%d ", n % 10);
}

int main()
{
   unsigned int num = 123;
   scanf("%d", &num);
   print(num);

}