C++ 代码
#include <iostream>
#include <string.h>
using namespace std;
string change(string num)
{
int sum = 0;
if(num.length() == 1) return num;
for (int i = 0; i < num.length(); ++i)
sum += stoi(num.substr(i,1));
return (change(to_string(sum)));
}
int main()
{
string num;
cin >> num;
cout << change(num);
}