# Successor Graphs

## Introduction

A Successor Graph is a model of a directed graph in which each node stores a list of the nodes that come after it. Successor graphs are better than an adjacency matrix or list because they speed up access to outgoing edges. This makes them perfect for algorithms that need quick access to successor vertices. This choice of design works well for graphs with a lot of points but not many edges

## Representation of Successor Graphs using Adjacency Matrix

Successor graphs store only the direct successors of each vertex, reducing memory usage and speeding up edge insertion and deletion operations in dense graphs with high out degrees

### Adjacency List vs. Successor Graph A Comparison

Adjacency lists can be memory-intensive for dense graphs; successor graphs optimize memory usage and speed up edge manipulation but may perform slower predecessor queries

### Converting an Adjacency List to a Successor Graph

Converting adjacency list to successor graph involves identifying direct successors, organizing in compact data structure, and iterating through list, retaining connectivity information, and reducing memory overhead.

## Successor Graph Operations

• Adding a Vertex and its Successors −

• When adding a new vertex to a successor graph, we must also define its direct successors. By updating the appropriate data structure, the new vertex can be efficiently integrated into the graph, allowing for easy access to its successors during traversal and other graph-related operations.

• Removing a Vertex and Updating Successors −

• When a vertex is removed from the successor graph, we need to adjust the successors of other vertices that were connected to the removed vertex. This ensures that the graph remains consistent and avoids potential issues during subsequent operations and traversals

• Finding Successors of a Given Vertex −

• Given a specific vertex in the successor graph, finding its direct successors is a crucial operation. With the graph's compact representation, this process becomes faster and more efficient, enabling rapid access to adjacent vertices for performing various graph algorithms.

• Determining Predecessors Using Reverse Successor Graph −

• Since successor graphs prioritize direct successors, determining predecessors becomes less efficient. However, a reverse successor graph can be created, where each vertex's successors become its predecessors. By applying standard successor graph operations to this reversed structure, we can efficiently find the predecessors of a given vertex.

### Traversal Algorithms on Successor Graphs

Traversal algorithms are essential for exploring graphs efficiently, and on successor graphs, they enable efficient traversal through direct successors.

• Depth-First Search (DFS) −

• DFS is a graph traversal algorithm that explores as far as possible along each branch before backtracking, and when applied to successor graphs, it efficiently navigates through direct successors, enabling various graph-related applications

### Algorithm

DFS(graph, start_vertex):
Create a stack data structure
Create a set to keep track of visited vertices

Push start_vertex to the stack
Mark start_vertex as visited

while the stack is not empty:
current_vertex = pop from the stack

for each neighbor in graph[current_vertex]:
if neighbor is not in the visited set:
Push neighbor to the stack
Mark neighbor as visited

• BFS is a graph traversal algorithm that explores all the vertices at the present depth before moving on to the next level, and when utilized on successor graphs, it effectively explores direct successors, offering useful insights into connected components and shortest paths.

### Algorithm

BFS(graph, start_vertex):
Create a queue data structure
Create a set to keep track of visited vertices

Enqueue start_vertex to the queue
Mark start_vertex as visited

while the queue is not empty:
current_vertex = dequeue from the queue

for each neighbor in graph[current_vertex]:
if neighbor is not in the visited set:

Enqueue neighbor to the queue
Mark neighbor as visited

Both DFS and BFS have

• Time-Complexity − O(V + E)

• Space Complexity − O(V)

Where, V represents the number of vertices and E represents the number of edges in the graph being processed

### Applications of Successor Graphs

• Topological Sorting

• Topological sorting is a crucial algorithm used in tasks that require a specific order of execution without any cyclic dependencies. Successor graphs offer an efficient representation for topological sorting. The algorithm involves repeatedly selecting vertices with no incoming edges and removing them from the graph. Successor graphs allow quick identification of vertices with no predecessors, simplifying the sorting process.

### Real-world examples and use cases

• Task scheduling: Successor graphs can be utilized to schedule tasks in project management, ensuring that tasks dependent on others are executed in the correct order.

• Compiler optimization: In compilers, topological sorting helps to determine the optimal order for code generation, reducing unnecessary dependencies

• Cycle Detection

• Detecting cycles in a graph is vital in numerous graph-related problems like deadlock detection, resource allocation, and more. Successor graphs can efficiently identify cycles in a graph, as cycles represent loops in the data structure.

### Importance in various graph-related problems

• Deadlock detection: Successor graphs can help identify circular dependencies among resources, which could lead to system deadlock

• Resource allocation: Detecting cycles in resource allocation graphs can prevent infinite loops and ensure proper resource distribution.

• Shortest Path Algorithms

• Successor graphs play a significant role in implementing shortest path algorithms, such as Dijkstra's algorithm and the Bellman-Ford algorithm

• Dijkstra's algorithm using successor graphs

• In Dijkstra's algorithm, successor graphs help in efficiently finding the shortest paths from a single source vertex to all other vertices in a weighted graph. The algorithm iteratively selects the vertex with the smallest distance from the source, and with successor graphs, these operations can be performed more efficiently

• Bellman-Ford algorithm with successor graph representation

• The Bellman-Ford algorithm can also be optimized using successor graphs. This algorithm finds the shortest paths from a single source vertex to all other vertices, even in graphs with negative edge weights. Successor graphs can streamline the process of relaxing edges and updating the shortest path information.

• Determining if there exists a path between two vertices

• Successor graphs enable the quick verification of whether there is a path between two given vertices. By performing a search for one vertex and checking if the other is reachable, we can determine the existence of a path

## Conclusion

In conclusion, Successor graphs prove to be a valuable representation in data structures for graph-related problems. Their efficient handling of topological sorting, cycle detection, shortest path algorithms, and reach ability makes them essential tools. With their versatility, Successor graphs contribute to optimizing graph-based applications and facilitating problem solving in various domains.

Someswar Pal

Studying Mtech/ AI- ML

Updated on: 28-Jul-2023

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