feat: add Swift codes for backtracking_algorithm article (#480)

* fix: compile error

* fix: package define

* feat: add Swift codes for backtracking_algorithm article

codes/swift/chapter_backtracking/preorder_traversal_ii_compact.swift

@@ -0,0 +1,51 @@
+/**
+ * File: preorder_traversal_ii_compact.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+var path: [TreeNode] = []
+var res: [[TreeNode]] = []
+
+/* 前序遍历：例题二 */
+func preOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    // 尝试
+    path.append(root)
+    if root.val == 7 {
+        // 记录解
+        res.append(path)
+    }
+    preOrder(root: root.left)
+    preOrder(root: root.right)
+    // 回退
+    path.removeLast()
+}
+
+@main
+enum PreorderTraversalIICompact {
+    /* Driver Code */
+    static func main() {
+        let root = TreeNode.listToTree(list: [1, 7, 3, 4, 5, 6, 7])
+        print("\n初始化二叉树")
+        PrintUtil.printTree(root: root)
+
+        // 前序遍历
+        path = []
+        res = []
+        preOrder(root: root)
+
+        print("\n输出所有根节点到节点 7 的路径")
+        for path in res {
+            var vals: [Int] = []
+            for node in path {
+                vals.append(node.val)
+            }
+            print(vals)
+        }
+    }
+}