正儿八经递归写法，一看就会，清晰明了
The Biggest Water Problem
https://www.nowcoder.com/practice/435aa556e093463891f6da7d322140a4
#include <stdio.h>
int fun(long long n)
{
int sum=0;
while(n)
{
sum+=n%10;
n/=10;
}
if(sum>9)
{
sum=fun(sum);
}
return sum;
}
int main()
{
long long n=0;
scanf("%lld",&n);
printf("%d\n",fun(n));
return 0;
}
