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+* graph is a non-linear data structure consisting of vertices and edges.
+* in **undirected graphs**, the edges between any two vertices do not have a direction, indicating a two-way relationship.
+* in **directed graphs**, the edges between any two vertices are directional.
+* in **weighted graphs**, each edge has an associated weight. if the sum of the weights of all edges of a cycle is a negative values, it's a negative weight cycle.
+* the **degree of a vertex** is the number of edges connecting the vertex. in directed, graphs, if the **in-dregree** of a vertex is d, there are **d** directional edges incident to the vertex (and similarly, **out-degree** from the vertex).
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+---
### traversals