Merge pull request #279 from nuomi1/feature/binary_tree_traversal-Swift

feat: add Swift codes for binary_tree_traversal article

docs/chapter_tree/binary_tree_traversal.md

@@ -188,7 +188,24 @@ comments: true
 === "Swift"
 
     ```swift title="binary_tree_bfs.swift"
-
+    /* 层序遍历 */
+    func hierOrder(root: TreeNode) -> [Int] {
+        // 初始化队列，加入根结点
+        var queue: [TreeNode] = [root]
+        // 初始化一个列表，用于保存遍历序列
+        var list: [Int] = []
+        while !queue.isEmpty {
+            let node = queue.removeFirst() // 队列出队
+            list.append(node.val) // 保存结点
+            if let left = node.left {
+                queue.append(left) // 左子结点入队
+            }
+            if let right = node.right {
+                queue.append(right) // 右子结点入队
+            }
+        }
+        return list
+    }
     ```
 
 ## 前序、中序、后序遍历
@@ -452,7 +469,38 @@ comments: true
 === "Swift"
 
     ```swift title="binary_tree_dfs.swift"
+    /* 前序遍历 */
+    func preOrder(root: TreeNode?) {
+        guard let root = root else {
+            return
+        }
+        // 访问优先级：根结点 -> 左子树 -> 右子树
+        list.append(root.val)
+        preOrder(root: root.left)
+        preOrder(root: root.right)
+    }
 
+    /* 中序遍历 */
+    func inOrder(root: TreeNode?) {
+        guard let root = root else {
+            return
+        }
+        // 访问优先级：左子树 -> 根结点 -> 右子树
+        inOrder(root: root.left)
+        list.append(root.val)
+        inOrder(root: root.right)
+    }
+
+    /* 后序遍历 */
+    func postOrder(root: TreeNode?) {
+        guard let root = root else {
+            return
+        }
+        // 访问优先级：左子树 -> 右子树 -> 根结点
+        postOrder(root: root.left)
+        postOrder(root: root.right)
+        list.append(root.val)
+    }
     ```
 
 !!! note