#include <optional>
#include <unordered_map>
#include <vector>

std::optional<int> oddOccurrencesInArray(const std::vector<int>& a)
{
    std::unordered_map<int, int> count;
    std::vector<int> order; // preserves first-seen order, like PHP's insertion-ordered array

    for (int value : a) {
        auto it = count.find(value);
        if (it == count.end()) {
            count[value] = 1;
            order.push_back(value);
        } else {
            count.erase(it);
        }
    }

    for (int v : order) {
        if (count.find(v) != count.end()) {
            return v;
        }
    }

    return std::nullopt;
}