题解 | #The Biggest Water Problem#

int output(int n) {
    while (1) {
        n = n % 10 + n / 10;
        if (n >= 10) output(n);
        else return n;
    }
}
int main() {
    int n, sum;
    scanf("%d", &n);
    printf("%d", output(n));
    return 0;
}