一些算法反向读取序列，所以要求双向迭代器。

Some read the sequence backward, thus requiring bidirectional iterators.

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下列范例使用位元组指标，在来源和目的字串之间读取及写入字元，借以反向复制字串。

The following example copies a string in reverse, by using byte Pointers to read and write characters between the source and destination strings.

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