Update array_binary_tree.cpp

[sslmj2020]

打印的小错误

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[sslmj2020]

比较的为值，非索引

[Justjustifyjudge]

确实如此，原本判断当前节点是否为nullptr，应该根据这个节点是否有合法值来进行判断，而不是根据这个节点在数组中的index判断

[Justjustifyjudge]

确实如此，原本判断当前节点是否为nullptr，应该根据这个节点是否有合法值来进行判断，而不是根据这个节点在数组中的index判断

[krahets]

Thanks to @sslmj2020 for the fix and to @Justjustifyjudge for the review!