Adjacency list in Swift

adjacency-list.swift

// Found this lying around in my code folder from when I was
// first playing with Swift. Consider it public domain.

struct Graph<T: Hashable> {
    let directed: Bool
    var adj = Dictionary<T, [T]>()

    init(directed d: Bool) {
        directed = d
    }

    mutating func addVertex(v: T) {
        self[v] = [T]()
    }

    mutating func addEdge(from v: T, to u: T) {
        if self[v] == nil {
            addVertex(v)
        }

        if self[u] == nil {
            addVertex(u)
        }

        self[v]?.append(u)

        if !directed {
            self[u]?.append(v)
        }
    }

    subscript(key: T) -> [T]? {
        get {
            return adj[key]
        }
        set(newValue) {
            return adj[key] = newValue
        }
    }
}


// Example usage:

var graph = Graph<Int>(directed: true)

graph.addEdge(from: 1, to: 2)
graph.addEdge(from: 1, to: 3)

graph[1] // => [2,3]

graph.addVertex(4)

graph[4] // => []

graph.addEdge(from: 2, to: 4)

graph[2] // => [4]
graph[4] // => []

graph.addEdge(from: 3, to: 3)

graph[3] // => [3]

graph[8] // => nil