Add kotlin code for the chapter of graph (#1102)

* feat(kotlin): add kotlin code for dynamic programming.

* Update knapsack.kt

* feat(kotlin): add kotlin codes for graph.

* style(kotlin): reformatted the codes.

codes/kotlin/chapter_graph/graph_dfs.kt

@@ -0,0 +1,62 @@
+/**
+ * File: graph_dfs.kt
+ * Created Time: 2024-01-25
+ * Author: curtishd (1023632660@qq.com)
+ */
+
+package chapter_graph
+
+import utils.Vertex
+
+/* 深度优先遍历辅助函数 */
+fun dfs(
+    graph: GraphAdjList,
+    visited: MutableSet<Vertex?>,
+    res: MutableList<Vertex?>,
+    vet: Vertex?
+) {
+    res.add(vet) // 记录访问顶点
+    visited.add(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  // 跳过已被访问的顶点
+        // 递归访问邻接顶点
+        dfs(graph, visited, res, adjVet)
+    }
+}
+
+/* 深度优先遍历 */
+// 使用邻接表来表示图，以便获取指定顶点的所有邻接顶点
+fun graphDFS(
+    graph: GraphAdjList,
+    startVet: Vertex?
+): List<Vertex?> {
+    // 顶点遍历序列
+    val res: MutableList<Vertex?> = ArrayList()
+    // 哈希表，用于记录已被访问过的顶点
+    val visited: MutableSet<Vertex?> = HashSet()
+    dfs(graph, visited, res, startVet)
+    return res
+}
+
+/* Driver Code */
+fun main() {
+    /* 初始化无向图 */
+    val v = Vertex.valsToVets(intArrayOf(0, 1, 2, 3, 4, 5, 6))
+    val edges = arrayOf(
+        arrayOf(v[0], v[1]),
+        arrayOf(v[0], v[3]),
+        arrayOf(v[1], v[2]),
+        arrayOf(v[2], v[5]),
+        arrayOf(v[4], v[5]),
+        arrayOf(v[5], v[6])
+    )
+    val graph = GraphAdjList(edges)
+    println("\n初始化后，图为")
+    graph.print()
+
+    /* 深度优先遍历 */
+    val res = graphDFS(graph, v[0])
+    println("\n深度优先遍历（DFS）顶点序列为")
+    println(Vertex.vetsToVals(res))
+}