这与二维的情形相同。
It is the same situation as when we had a circle in the 2d case.
第二种情形，即变量的条件修改，是重新使用的问题的一个子集，只是有时我们会保持现有的值，有时需要使用一个新值。
The second case, the conditional modification of a variable, is a subset of the variable reuse problem except that sometimes we will keep our existing value and sometimes we will want a new value.
对于第二次故障，情形1：如果A出现故障，它将被转移到空闲节点上。
For the second failure, case 1: If a fails, it is placed on the spare node.
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