Update array_deque.cpp函数index()

[KLYkl]

将43行的return (i + capacity()) % capacity();改为return (i % capacity() + capacity()) % capacity()； 原因：原来的表达式“(i + capacity()) % capacity();”如果i是负数的话，需要保证capacity() >= -i。而更改后的表达式“ (i % capacity() + capacity()) % capacity();” 中的先对i取模相加，最后再取模操作会确保结果是一个非负整数，并且不会超过 capacity() 的值。这是因为取模操作会返回一个在 0 到 capacity() - 1 范围内的数，即使原始的和可能是负数。保证函数index()的传入形参i<-capacity()不会崩溃，且能正确返回 0 到 capacity() - 1 范围内的的索引值。

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[krahets]

Hi，i 定义为索引，因此输入应该在区间 [0, capacity()) 内，不应出现负数的情况。

严谨的做法是在 index() 方法中添加安全判断，保证输入参数 i 满足索引范围。但为了简洁性，书中代码会省略部分安全判断等代码。

[KLYkl]

Hi， 定义为索引，因此输入应该在区间 内，不应出现负数的情况。i``[0, capacity())

严谨的做法是在 方法中添加安全判断，保证输入参数 满足索引范围。但为了简洁性，书中代码会省略部分安全判断等代码。index()``i

这么说那i%capacity()就可以了，不必写（i + capacity（）） % capacity（）了吧？