Graph data structure in Swift using an Adjacency List.

GraphAL.swift

public struct GraphAL<Element: Hashable> {
    var adjList = [Element: GraphNode<Element>]()
    
    public mutating func addVertex(_ name: Element) {
        adjList[name] = GraphNode(name)
    }

    @discardableResult public mutating func addEdge(from source: Element, to destination: Element) -> Bool {
        guard adjList[source] != nil, adjList[destination] != nil
            else { return false }
        
        var node = GraphNode(destination)
        node.next = adjList[source]
        adjList[source] = node
        
        node = GraphNode(source)
        node.next = adjList[destination]
        adjList[destination] = node
        
        return true
    }
}

GraphNode.swift

public class GraphNode<Element: Hashable> {
    public var data: Element
    public var next: GraphNode<Element>?
    
    init(_ element: Element) {
        self.data = element
    }
}