#include <cassert>
#include <unordered_set>
#include <vector>

class Solution {
  public:
    std::vector<int> findErrorNums(const std::vector<int> &nums) {
        int expected_sum = nums.size() * (1 + nums.size()) / 2;
        int sum = 0;
        int unique_sum = 0;

        std::unordered_set<int> seen;
        seen.reserve(nums.size());
        for (const int x : nums) {
            sum += x;
            if (!seen.contains(x)) {
                unique_sum += x;
            }
            seen.insert(x);
        }

        return {sum - unique_sum, expected_sum - unique_sum};
    }
};

int main() {
    Solution s;

    assert((s.findErrorNums(std::vector{1, 2, 2, 4}) == std::vector{2, 3}));
    assert((s.findErrorNums(std::vector{1, 1}) == std::vector{1, 2}));

    return 0;
}