0001 template <typename T> TowerPosi<T> Skiplist<T>::search(const T& e) {
0002    _x = head; //从head塔的顶层出发
0003    for (Rank h = height(_x) - 1; -1 != h; h--) //逐层下降，直至底层
0004       while ((tower(_x)[h].succ != tail) && (key(tower(_x)[h].succ) <= e)) //在每一层
0005          _x = tower(_x)[h].succ; //都尽可能地右移
0006    return _x == head ? nullptr : _x; //最终止步于不大于e的最后一座塔
0007 } //search