该清单展示了内部迭代器(也称为push迭代器，因为它将每个元素依次推到代码块中)。

This listing illustrates an internal iterator (also known as a push iterator because it pushes each element in turn to the code block).

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但是向后移动要复杂些，因为迭代器内部执行的是深度优先算法，预先排好了Rope遍历的顺序，需要访问每个叶节点。

But moving backward is more complicated, because internally the iterator performs a depth-first, preorder traversal of the Rope, visiting each of its leaf nodes.

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迭代器模式：提供一种方法顺序访问一个聚合对象中的各个元素，而又不暴露其内部的表示。

The Iterator Pattern:provides a way to access the elements of an aggregate object sequentially without exposing its underying representation.

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迭代器模式：提供一种方法顺序访问一个聚合对象中的各个元素，而又不暴露其内部的表示。

The Iterator Pattern:provides a way to access the elements of an aggregate object sequentially without exposing its underying representation.

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