题解 | The Biggest Water Problem

#include <stdio.h>

long long ADD(long long a){
    long long sum=0;
    while(a){
        sum += a%10;
        a /= 10;
    }
    return sum;
}

int cycle(long long m){
    if(ADD(m)<=9){
        return ADD(m);
    }
    else{
        return cycle(ADD(m));
    }
}
int main() {
    long long n;
    scanf("%lld",&n);
    long long sum = cycle(n);
    printf("%lld",sum);
    return 0;
}