0001 template <typename T> TowerPosi<T> Skiplist<T>::insert(const T& e) {
0002    search(e); if ((_x != head) && (e == key(_x))) return _x; //禁止重复
0003    TowerPosi<T> pred = _x, succ = _x->succ; //同层的前驱（或为head）、后继（或为tail）
0004    _x = List::insert(_x, Tower<T>(e)); //紧邻于_x之右侧，创建一座空塔
0005    for (Rank h = 0; !h || (rand()&1); h++) { //积砖成塔（至少一层）
0006       if (height(head) < h+1) { //若总高度不足h
0007          tower(head).insert(Brick<T>(nullptr, tail)); //则需创建
0008          tower(tail).insert(Brick<T>(head, nullptr)); //一层空表
0009       } //if
0010       while (height(pred) == h) //踏过若干塔顶，找到前驱
0011          pred = tower(pred)[h-1].pred;
0012       while (height(succ) == h) //踏过若干塔顶，找到后继
0013          succ = tower(succ)[h-1].succ;
0014       tower(_x).insert(Brick<T>(pred, succ)); //新塔长高一层，并
0015       tower(pred)[h].succ = tower(succ)[h].pred = _x; //与前驱、后继连接
0016    } //for
0017    return _x; //返回新塔
0018 } //insert