Address comments and refactor

Author: Xiangxin

Minimum Spanning Tree/MinimumSpanningTree.playground/Contents.swift

@@ -30,15 +30,15 @@ func minimumSpanningTreePrim<T>(graph: Graph<T>) -> (cost: Int, tree: Graph<T>)
   var cost: Int = 0
   var tree = Graph<T>()
 
-  if graph.vertices.isEmpty {
+  guard let start = graph.vertices.first else {
     return (cost: cost, tree: tree)
   }
 
   var visited = Set<T>()
   var priorityQueue = PriorityQueue<(vertex: T, weight: Int, parent: T?)>(
     sort: { $0.weight < $1.weight })
 
-  priorityQueue.enqueue((vertex: graph.vertices.first!, weight: 0, parent: nil))
+  priorityQueue.enqueue((vertex: start, weight: 0, parent: nil))
   while let head = priorityQueue.dequeue() {
     let vertex = head.vertex
     if visited.contains(vertex) {

Minimum Spanning Tree/MinimumSpanningTree.playground/Sources/Graph.swift

@@ -36,16 +36,17 @@ public struct Graph<T: Hashable>: CustomStringConvertible {
     edgeList.append(Edge(vertex1: v1, vertex2: v2, weight: w))
     vertices.insert(v1)
     vertices.insert(v2)
-    if adjList[v1] == nil {
-      adjList[v1] = [(vertex: v2, weight: w)]
+
+    if let _ = adjList[v1] {
+      adjList[v1]?.append((vertex: v2, weight: w))
     } else {
-      adjList[v1]!.append((vertex: v2, weight: w))
+      adjList[v1] = [(vertex: v2, weight: w)]
     }
 
-    if adjList[v2] == nil {
-      adjList[v2] = [(vertex: v1, weight: w)]
+    if let _ = adjList[v2] {
+      adjList[v2]?.append((vertex: v1, weight: w))
     } else {
-      adjList[v2]!.append((vertex: v1, weight: w))
+      adjList[v2] = [(vertex: v1, weight: w)]
     }
   }
 

Minimum Spanning Tree/MinimumSpanningTree.playground/playground.xcworkspace/contents.xcworkspacedata

@@ -10,15 +10,15 @@ func minimumSpanningTreePrim<T>(graph: Graph<T>) -> (cost: Int, tree: Graph<T>)
   var cost: Int = 0
   var tree = Graph<T>()
 
-  if graph.vertices.isEmpty {
+  guard let start = graph.vertices.first else {
     return (cost: cost, tree: tree)
   }
 
   var visited = Set<T>()
   var priorityQueue = PriorityQueue<(vertex: T, weight: Int, parent: T?)>(
     sort: { $0.weight < $1.weight })
 
-  priorityQueue.enqueue((vertex: graph.vertices.first!, weight: 0, parent: nil))
+  priorityQueue.enqueue((vertex: start, weight: 0, parent: nil))
   while let head = priorityQueue.dequeue() {
     let vertex = head.vertex
     if visited.contains(vertex) {
@@ -43,3 +43,4 @@ func minimumSpanningTreePrim<T>(graph: Graph<T>) -> (cost: Int, tree: Graph<T>)
 
   return (cost: cost, tree: tree)
 }
+