题解 | #Digital Roots#

#include <stdio.h>

int main() {
    int a;
    while (scanf("%d", &a) != EOF) { // 注意 while 处理多个 case
        while (a > 9) {
            int sum = 0;//a各位数字之和
            while (a > 0) {
                sum += a % 10;
                a /= 10;
            }
            a = sum;
        }
        printf("%d\n", a);
    }
    return 0;
}