In the previous post we introduced a graph data structure and implemented a basic graph model. In this very short post we are going to talk about adding an edge between two vertices as well as checking if an edge exists between vertices.

## Adding An Edge

Our graph is an undirected graph so when we add an edge to one vertex’s list we need to also do the same in the other vertex’s list. Before we do that we just need to make sure the vertices exist. Time complexity for this is \(O(1)\) – the reason why I used a hash map and a hash set. Here’s the code:

```
public void AddEdge(T source, T destination)
{
AddVertex(source);
AddVertex(destination);
_adjacentList[source].Add(destination);
_adjacentList[destination].Add(source);
}
```

## Checking If Edge Exists

Checking if an edge exists between two vertices can also be done in \(O(1)\) time. Here’s the code:

```
public bool HasEdge(T source, T destination)
{
if (!_adjacentList.ContainsKey(source))
{
return false;
}
return _adjacentList[source].Contains(destination);
}
```

This was a very short post where we spoke about how to add an edge between two vertices in a graph. All the code is available on GitHub if you want to check it out. Like always, thanks for reading.

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