题解 | The Biggest Water Problem

#include <stdio.h>
int main() {
    int a;
    scanf("%d", &a);
    int sum = 0;
    int k = 0;
    while (1) {
        sum = 0;
        while (a) {
            k = a % 10;
            sum += k;
            a = a / 10;
        }
        if (sum < 10) {
            printf("%d", sum);
            break;
        }
        a = sum;
    }
    return 0;
}