Q11332: Summing Digits http://luckycat.kshs.kh.edu.tw/homework/q11332.htm

#include<iostream>
using namespace std;

int main(){
  int n;
  while( cin>>n ){
    while( n/10 ) n = n/10 + n%10;
    cout<<n<<endl;
  }
  return 0;
}

遞迴

#include<iostream>
using namespace std;

int sumDigits(int n, int sum) {
    // Basic Case to stop the recursion
    if (n== 0)  {
        return sum;
    } else {
        sum = sum + n % 10;  //recursive variable to keep the digits sum
        n= n/10;
        return sumDigits(n, sum); //returning sum to print it.
    }
}

int main(){

    int n;

    while( cin>>n ){
        cout<<sumDigits(n,0)<<endl;
    }

}