1213. 三个有序数组的交集

vector arraysIntersection(vector &arr1, vector &arr2, vector &arr3) {
    map m1;
    unordered_map m2, m3;
    for (auto it:arr1)
        m1[it]++;

    for (auto it:arr2)
        m2[it]++;

    for (auto it:arr3)
        m3[it]++;

    vector ret;
    for (auto it:m1)
        if (m2.find(it.first) != m2.end() && m3.find(it.first) != m3.end())
            ret.emplace_back(it.first);
    return ret;
}