# Check if the given graph represents a Ring Topology

Given a graph G, the task is to check if it represents a Ring Topology.

A Ring Topology is the one shown in the image below: Examples:

```Input :  Graph = Output : YES

Input : Graph = Output : NO
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A graph of V vertices represents a Ring topology if it satisfies the following three conditions:

1. Number of vertices >= 3.
2. All vertices should have degree 2.
3. No of edges = No of Vertices.

The idea is to traverse the graph and check if it satisfies the above three conditions. If yes, then it represents a Ring Topology otherwise not.

Below is the implementation of the above approach:

## C++

 `// CPP program to check if the given graph ` `// represents a Ring topology ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// A utility function to add an edge in an ` `// undirected graph. ` `void` `addEdge(vector<``int``> adj[], ``int` `u, ``int` `v) ` `{ ` `    ``adj[u].push_back(v); ` `    ``adj[v].push_back(u); ` `} ` ` `  `// A utility function to print the adjacency list ` `// representation of graph ` `void` `printGraph(vector<``int``> adj[], ``int` `V) ` `{ ` `    ``for` `(``int` `v = 0; v < V; ++v) { ` `        ``cout << ``"\n Adjacency list of vertex "` `             ``<< v << ``"\n head "``; ` `        ``for` `(``auto` `x : adj[v]) ` `            ``cout << ``"-> "` `<< x; ` `        ``printf``(``"\n"``); ` `    ``} ` `} ` ` `  `/* Function to return true if the graph represented  ` `   ``by the adjacency list represents a Ring topology  ` `   ``else return false */` `bool` `checkRingTopologyUtil(vector<``int``> adj[], ``int` `V, ``int` `E) ` `{ ` `    ``// Number of edges should be equal ` `    ``// to Number of vertices ` `    ``if` `(E != V) ` `        ``return` `false``; ` ` `  `    ``// For a graph to represent a ring topology should have ` `    ``// greater than 2 nodes ` `    ``if` `(V <= 2) ` `        ``return` `false``; ` ` `  `    ``int``* vertexDegree = ``new` `int``[V + 1]; ` `    ``memset``(vertexDegree, 0, ``sizeof` `vertexDegree); ` ` `  `    ``// calculate the degree of each vertex ` `    ``for` `(``int` `i = 1; i <= V; i++) { ` `        ``for` `(``auto` `v : adj[i]) { ` `            ``vertexDegree[v]++; ` `        ``} ` `    ``} ` ` `  `    ``// countDegree2 stores the count of ` `    ``// the vertices having degree 2 ` `    ``int` `countDegree2 = 0; ` ` `  `    ``for` `(``int` `i = 1; i <= V; i++) { ` `        ``if` `(vertexDegree[i] == 2) { ` `            ``countDegree2++; ` `        ``} ` `    ``} ` ` `  `    ``// if all three necessary conditions as discussed, ` `    ``// satisfy return true ` `    ``if` `(countDegree2 == V) { ` `        ``return` `true``; ` `    ``} ` `    ``else` `{ ` `        ``return` `false``; ` `    ``} ` `} ` ` `  `// Function to check if the graph represents a Ring topology ` `void` `checkRingTopology(vector<``int``> adj[], ``int` `V, ``int` `E) ` `{ ` `    ``bool` `isRing = checkRingTopologyUtil(adj, V, E); ` `    ``if` `(isRing) { ` `        ``cout << ``"YES"` `<< endl; ` `    ``} ` `    ``else` `{ ` `        ``cout << ``"NO"` `<< endl; ` `    ``} ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``// Graph 1 ` `    ``int` `V = 6, E = 6; ` `    ``vector<``int``> adj1[V + 1]; ` `    ``addEdge(adj1, 1, 2); ` `    ``addEdge(adj1, 2, 3); ` `    ``addEdge(adj1, 3, 4); ` `    ``addEdge(adj1, 4, 5); ` `    ``addEdge(adj1, 6, 1); ` `    ``addEdge(adj1, 5, 6); ` `    ``checkRingTopology(adj1, V, E); ` ` `  `    ``// Graph 2 ` `    ``V = 5, E = 4; ` `    ``vector<``int``> adj2[V + 1]; ` `    ``addEdge(adj2, 1, 2); ` `    ``addEdge(adj2, 1, 3); ` `    ``addEdge(adj2, 3, 4); ` `    ``addEdge(adj2, 4, 5); ` `    ``checkRingTopology(adj2, V, E); ` ` `  `    ``return` `0; ` `} `

## Python3

 `# Python3 program to check if the given graph ` `# represents a star topology ` ` `  `# A utility function to add an edge in an ` `# undirected graph. ` ` `  `def` `addEdge(adj, u, v): ` `    ``adj[u].append(v) ` `    ``adj[v].append(u) ` ` `  `# A utility function to prthe adjacency list ` `# representation of graph ` `def` `printGraph(adj, V): ` ` `  `    ``for` `v ``in` `range``(V): ` `        ``print``(``"Adjacency list of vertex "``,v,``"\n head "``) ` `        ``for` `x ``in` `adj[v]: ` `            ``print``(``"-> "``,x,end``=``" "``) ` `        ``printf() ` ` `  `# /* Function to return true if the graph represented ` `#    by the adjacency list represents a ring topology ` `#    else return false */ ` `def` `checkRingTopologyUtil(adj, V, E): ` ` `  `    ``# Number of edges should be equal ` `    ``# to (Number of vertices - 1) ` `    ``if` `(E !``=` `(V)): ` `        ``return` `False` ` `  `    ``# For a graph to represent a ring topology should have ` `    ``# greater than 2 nodes ` `    ``if` `(V <``=` `2``): ` `        ``return` `False` ` `  `    ``vertexDegree ``=` `[``0``]``*``(V ``+` `1``) ` ` `  `    ``# calculate the degree of each vertex ` `    ``for` `i ``in` `range``(V``+``1``): ` `        ``for` `v ``in` `adj[i]: ` `            ``vertexDegree[v] ``+``=` `1` `     `  `    ``# countDegree2 stores the count of ` `    ``# the vertices having degree 2 ` `    ``countDegree2 ``=` `0` ` `  `    ``for` `i ``in` `range``(``1``, V ``+` `1``): ` `        ``if` `(vertexDegree[i] ``=``=` `2``): ` `            ``countDegree2 ``+``=` `1` ` `  `    ``# if all three necessary conditions as discussed, ` `    ``# satisfy return true ` `    ``if` `(countDegree2 ``=``=` `V): ` `        ``return` `True` `    ``else``: ` `        ``return` `False` ` `  `# Function to check if the graph represents a ring topology ` `def` `checkRingTopology(adj, V, E): ` ` `  `    ``isRing ``=` `checkRingTopologyUtil(adj, V, E) ` `    ``if` `(isRing): ` `        ``print``(``"YES"``) ` ` `  `    ``else``: ` `        ``print``(``"NO"` `) ` ` `  `# Driver code ` ` `  `# Graph 1 ` `V,E ``=` `6``,``6` `adj1 ``=` `[[] ``for` `i ``in` `range``(V ``+` `1``)] ` `addEdge(adj1, ``1``, ``2``) ` `addEdge(adj1, ``2``, ``3``) ` `addEdge(adj1, ``3``, ``4``) ` `addEdge(adj1, ``4``, ``5``) ` `addEdge(adj1, ``6``, ``1``) ` `addEdge(adj1, ``5``, ``6``) ` `checkRingTopology(adj1, V, E) ` ` `  `# Graph 2 ` `V,E ``=` `5``,``4` `adj2 ``=` `[[] ``for` `i ``in` `range``(V ``+` `1``)] ` `addEdge(adj2, ``1``, ``2``) ` `addEdge(adj2, ``1``, ``3``) ` `addEdge(adj2, ``3``, ``4``) ` `addEdge(adj2, ``4``, ``2``) ` `checkRingTopology(adj2, V, E) ` ` `  `# This code is contributed by mohit kumar 29 `

Output:

```YES
NO
```

Time Complexity: O(V + E) where V and E are the numbers of vertices and edges in the graph respectively. My Personal Notes arrow_drop_up Check out this Author's contributed articles.

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Improved By : mohit kumar 29