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fix typos: remove duplicated word in comments
**What this PR does / why we need it**: Remove the duplicated word `the` in comments
**Which issue this PR fixes** : fixes #
**Special notes for your reviewer**:
```release-note
NONE
```
Use visitors to avoid allocating slices to hold nodes during traversal (and to allow short-circuiting)
Benchmarked at 95% space savings traversing nodes with many edges.
Implements graph.Directed capable of storing at most one edge between any two nodes.
Uses the Undirected implementation for space efficiency (~30% space savings).