Algorithm - How to solve an arithmetic expression DAG?

I believe, to achieve the desired efficiency, the problem wants you to avoid re-evaluating parts of the tree you've already visited. Once you've reached and evaluated some sub-tree in the DAG (every node in the tree represents the sub-tree rooted at that node), you can store the resulting value and associate it with that sub-tree. Then, when you visit it again, you can check whether you've pre-computed that value and just retrieve it rather than evaluating it again.

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