# Detecting Cycles

Understand the different techniques used to detect cycles in a graph.

## We'll cover the following

## Directed acyclic graph

A **directed acyclic graph** or **dag** is a directed graph with no directed cycles. Any vertex in a dag that has no incoming vertices is called a **source**; any vertex with no outgoing edges is called a **sink**. An isolated vertex with no incident edges at all is both a source and a sink. Every dag has at least one source and one sink but may have more than one of each. For example, in the graph with $n$ vertices but no edges, every vertex is a source, and every vertex is a sink.

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