What Is Cycle in Graph Data Structure?


Larry Thompson

In graph theory, a cycle is a path in a graph that starts and ends at the same vertex. A cycle can be of any length, including zero (a self-loop) or one (an edge connecting a vertex to itself).

Types of Cycles

There are several types of cycles that can occur in a graph:

Simple Cycle

A simple cycle is a cycle where no vertex (except the first and last) appears more than once. In other words, it does not contain any repeated vertices or edges.


A self-loop is an edge that connects a vertex to itself. It forms a cycle of length 1.

Multi-Edge Cycle

A multi-edge cycle contains multiple edges between the same pair of vertices. These edges contribute to the length of the cycle.

Detecting Cycles

Detecting cycles in a graph is an important task in graph algorithms. There are several algorithms available for this purpose:

  • Breadth-First Search (BFS): BFS can be used to detect cycles in an undirected graph. Start traversing the graph from any vertex and keep track of visited vertices. If while traversing, an already visited vertex is encountered (other than its parent), then there exists a cycle.
  • Depth-First Search (DFS): DFS can be used to detect cycles in both directed and undirected graphs. Similar to BFS, keep track of visited vertices while traversing.

    If while exploring the neighbors of a vertex, an already visited vertex is encountered, then there exists a cycle present.

  • Union-Find Algorithm: The union-find algorithm can be used to detect cycles in an undirected graph. It uses a disjoint set data structure to keep track of connected components. If while adding an edge, the two vertices already belong to the same set, then adding that edge would create a cycle.

Applications of Cycles in Graphs

Cycles in graph data structures have various applications:

  • Graph Theory: Cycles play a fundamental role in graph theory and are used to define important concepts such as connectivity, spanning trees, and Eulerian paths.
  • Network Routing: In network routing algorithms, cycles can represent loops in the network topology that need to be avoided for efficient data transmission.
  • Deadlock Detection: Cycles are used in deadlock detection algorithms to identify circular dependencies between processes or resources.
  • Genetic Algorithms: Cycles are utilized in genetic algorithms for optimization problems where solutions evolve over generations through crossover and mutation operations.

In Conclusion

Cycles are an essential concept in graph theory and have various applications across different domains. Detecting cycles is crucial for understanding the structure and properties of graphs and is key to solving many graph-related problems. By identifying and utilizing cycles effectively, we can unlock valuable insights from graph data structures.

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