问有限项的两个数组之间的排列

EN

enum E { A, B, None };

std::vector<E> es {E::A, E::A, E::B, E::B};
es.resize(a.size(), E::None); // es is sorted

do {
    std::cout << "checking:";
    int index = -1;
    for (auto e : es) {
         ++index;
         switch (e) {
             case E::A: std::cout << " A :" << index;
             case E::B: std::cout << " B :" << index;
         }
    }
    std::cout << std::endl;
} while (std::next_permutation(es.begin(), es.end()));