#include <bits/stdc++.h>
using namespace std;
int num, ans;
int solve()
{
    ans = num;
    if (num < 10)
        return num;
    num = 0;
    while (ans)
    {
        num += ans % 10;
        ans /= 10;
    }
    solve();
}
int main()
{
    string s;
    cin >> s;
    for (int i = 0; i < s.length(); i++)
        ans += s[i] - '0';
    if (ans < 10)
        cout << ans << endl;
    else
    {
        num = ans;
        cout << solve() << endl;
    }
    return 0;
}

已AC，放心食用

#include <bits/stdc++.h> 
using namespace std;
int shift(int n)
{
    int num = 0;
    while(n > 0)
    {
        num += n % 10;
        n /= 10;
    }
    return num;
}
int main()
{
    long long n;
    cin >> n;
    while(n / 10 != 0)
    {
        n = shift(n);
    }
    cout << n;
    return 0;
}