0001 template <typename T> LNP<T> List<T>::join(List<T>& L) { //L作为后缀接入当前列表
0002    if (L._size < 1) return nullptr;
0003    LNP<T> first = L.head->succ;
0004    tail->pred->succ = L.head->succ;
0005    L.head->succ->pred = tail->pred;
0006    L.tail->pred->succ = tail;
0007    tail->pred = L.tail->pred;
0008    _size += L._size;
0009    L.head->succ = L.tail;
0010    L.tail->pred = L.head;
0011    L._size = 0;
0012    return first;
0013 } //O(1)