第20行访问栈顶(栈顶的索引为- 1)作为lua字符串，打印消息，然后从堆栈中删除该值。

Line 20 accesses the top of the stack (the top of the stack has an index of -1) as a Lua string, prints the message, and then removes the value from the stack.

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对于输入的每一对字符串，你的程序应该按顺序生成所有的有效的进出栈操作列表，其中的每组操作都能使原字符串转变为目标字符串。

For each input pair, your program should produce a sorted list of valid sequences of I and o which produce the target word from the source word.

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最后，当整个字符串处理完毕并且同时栈被清空，字符串就是完全平衡的。

Once again, if the entire string isprocessed and nothing is left on the stack, the string is correctly balanced.

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主要包括的内容有：线性表，栈与队列，字符串，二叉树，树，图。

We will introduce Linear Lists, Stacks, Queues, Strings, Binary Trees, Trees and Graphs, which are fundamental data structures.

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主要包括的内容有：线性表，栈与队列，字符串，二叉树，树，图。

We will introduce Linear Lists, Stacks, Queues, Strings, Binary Trees, Trees and Graphs, which are fundamental data structures.

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