feat: add Swift codes for graph_traversal article (#378)

* feat: add Swift codes for graph_traversal article

* refactor: rename parameters

* Update graph_dfs.swift

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/swift/chapter_graph/graph_dfs.swift

@@ -0,0 +1,54 @@
+/**
+ * File: graph_dfs.swift
+ * Created Time: 2023-02-21
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import graph_adjacency_list_target
+import utils
+
+/* 深度优先遍历 DFS 辅助函数 */
+func dfs(graph: GraphAdjList, visited: inout Set<Vertex>, res: inout [Vertex], vet: Vertex) {
+    res.append(vet) // 记录访问顶点
+    visited.insert(vet) // 标记该顶点已被访问
+    // 遍历该顶点的所有邻接顶点
+    for adjVet in graph.adjList[vet] ?? [] {
+        if visited.contains(adjVet) {
+            continue // 跳过已被访问过的顶点
+        }
+        // 递归访问邻接顶点
+        dfs(graph: graph, visited: &visited, res: &res, vet: adjVet)
+    }
+}
+
+/* 深度优先遍历 DFS */
+// 使用邻接表来表示图，以便获取指定顶点的所有邻接顶点
+func graphDFS(graph: GraphAdjList, startVet: Vertex) -> [Vertex] {
+    // 顶点遍历序列
+    var res: [Vertex] = []
+    // 哈希表，用于记录已被访问过的顶点
+    var visited: Set<Vertex> = []
+    dfs(graph: graph, visited: &visited, res: &res, vet: startVet)
+    return res
+}
+
+@main
+enum GraphDFS {
+    /* Driver Code */
+    static func main() {
+        /* 初始化无向图 */
+        let v = Vertex.valsToVets(vals: [0, 1, 2, 3, 4, 5, 6])
+        let edges = [
+            [v[0], v[1]], [v[0], v[3]], [v[1], v[2]],
+            [v[2], v[5]], [v[4], v[5]], [v[5], v[6]],
+        ]
+        let graph = GraphAdjList(edges: edges)
+        print("\n初始化后，图为")
+        graph.print()
+
+        /* 深度优先遍历 DFS */
+        let res = graphDFS(graph: graph, startVet: v[0])
+        print("\n深度优先遍历（DFS）顶点序列为")
+        print(Vertex.vetsToVals(vets: res))
+    }
+}