What Is Adjacency Graph in Data Structure?


Heather Bennett

When it comes to data structures, one important concept to understand is the adjacency graph. An adjacency graph is a representation of connections or relationships between different elements in a data set. It is widely used in computer science and plays a crucial role in various algorithms and applications.

What is an Adjacency Graph?

An adjacency graph, also known as an adjacency list or an adjacency matrix, is a way to represent connections between vertices or nodes in a graph. In simple terms, it shows which nodes are adjacent to each other in a graph.

In an adjacency list representation, each node of the graph maintains a list of its adjacent nodes. This can be implemented using arrays, linked lists, or other data structures. On the other hand, an adjacency matrix uses a two-dimensional array to represent the connections between nodes.

Why Use Adjacency Graphs?

The use of adjacency graphs provides several benefits:

  • Efficient Representation: Adjacency graphs provide an efficient representation of connections between nodes in a graph. They are particularly useful when dealing with sparse graphs where only a few connections exist between nodes.
  • Easy Traversal: With the help of adjacency graphs, it becomes easier to traverse through all the neighboring nodes of a particular node.

    This is especially important when implementing algorithms like breadth-first search (BFS) or depth-first search (DFS).

  • Space Efficiency: Depending on the density and size of the graph, one representation may be more space-efficient than the other. For example, if there are fewer edges compared to the number of nodes in the graph, an adjacency list can save memory by not storing unnecessary information.

Adjacency List vs Adjacency Matrix

Both adjacency list and adjacency matrix have their own advantages and disadvantages.

Adjacency List:

  • Space Efficiency: Adjacency lists are more space-efficient when dealing with sparse graphs as they only store connections that exist.
  • Easier to Traverse: Traversing through adjacent nodes is faster in an adjacency list as it directly accesses the connected nodes.
  • Inefficient for Dense Graphs: If the graph is dense, i.e., there are many edges compared to the number of nodes, an adjacency list might be less efficient as it requires traversing multiple lists to find connections.

Adjacency Matrix:

  • Easier Edge Existence Check: Adjacency matrices make it easier to check whether an edge exists between two nodes by simply accessing the corresponding matrix cell.
  • Faster Edge Removal: Removing an edge from an adjacency matrix is faster compared to an adjacency list as it only requires updating a single cell.
  • Inefficient for Sparse Graphs: Adjacency matrices consume more memory for sparse graphs as they represent all possible connections, including those that don’t exist.


An adjacency graph provides a convenient way to represent the connections between nodes in a graph. Whether using an adjacency list or an adjacency matrix depends on the specific requirements of the application and the characteristics of the graph.

Both representations have their own advantages and trade-offs in terms of space efficiency and traversal speed. Understanding these concepts is essential for implementing efficient algorithms and solving problems related to graph theory.

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