0001 template <typename T> //0 <= lo < hi <= _size
0002 Rank Vector<T>::find(const T& e, Rank lo, Rank hi) const { //O(hi-lo)
0003    T bak = _elem[hi];  _elem[hi] = e; //先将目标存入哨兵：因总有_size < _capa，故不致溢出
0004    while (e != _elem[lo]) lo++; //再从前向后，逐个比对
0005    _elem[hi] = bak; return lo; //哨兵还原；返回最靠前的命中者，或hi示意失败
0006 } //find