使用了一个有序的Set

#include <iostream>
#include <vector>
#include <set>

using namespace std;
int n;

int sum_digits(int x) {
  int s = 0;
  while (x) {
    s += x % 10;
    x /= 10;
  }
  return s;
}

int main(int argc, char** argv) {
  cin >> n;
  vector<int> nums(n);
  set<int> friends;

  for (int i = 0; i < n; ++i) cin >> nums[i];

  for (const auto& x : nums)
    friends.emplace(sum_digits(x));

  vector<int> ans(begin(friends), end(friends));
  printf("%d\n", ans.size());
  for (int i = 0; i < ans.size(); ++i) {
    printf("%d", ans[i]);
    if (i < ans.size() - 1) putchar(' ');
  }

  return 0;
}