#include<stdio.h>
int fun(int s1,int s2){
    if(s2<10)
    return(s1*10+s2%10);
    s1=s1*10+s2%10;
    s1=fun(s1,s2/10);
    return s1;
}
int main(){
    printf("%d",fun(0,1234));
}

算法没变，只是增加了一条输出语句

#include <stdio.h>

unsigned fun( unsigned a, unsigned b )
{
    printf( "%u, %u\n", a, b );
    if( b == 0 ) 
        return a;
    return fun( a*10+b%10, b/10 );
}

int main( void )
{
    printf( "%u\n", fun(12345,678) );
}

输出是
12345, 678
123458, 67
1234587, 6
12345876, 0
12345876
看出来了吧

供参考：

#include<stdio.h>
int fun(int s1, int s2) {
    printf("%d,%d\n", s1, s2);
    if (s2 < 10)
        return(s1 * 10 + s2 % 10);
    s1 = s1 * 10 + s2 % 10;
    s1 = fun(s1, s2 / 10);
    return s1;
}
int main() {
    printf("%d", fun(0, 12345));
    return 0;
}
//fun()函数为递归函数，实现将数 s2 逆序后，保存到 s1. 
//s1   s2 
//0, 12345
//5, 1234
//54, 123
//543, 12
//5432, 1
//54321

[此贴子已经被作者于2022-2-25 13:42编辑过]