二叉树的迭代遍历：补充Swift实现

# Conflicts:
#	problems/二叉树的迭代遍历.md

problems/二叉树的迭代遍历.md

@@ -390,7 +390,7 @@ func inorderTraversal(root *TreeNode) []int {
 }
 ```
 
-javaScript
+javaScript：
 
 ```js
 
@@ -454,7 +454,7 @@ var postorderTraversal = function(root, res = []) {
 };
 ```
 
-TypeScript:
+TypeScript：
 
 ```typescript
 // 前序遍历（迭代法）
@@ -509,77 +509,63 @@ function postorderTraversal(root: TreeNode | null): number[] {
 };
 ```
 
-Swift:
+Swift：
 
-> 迭代法前序遍历
 ```swift
+// 前序遍历迭代法
 func preorderTraversal(_ root: TreeNode?) -> [Int] {
-    var res = [Int]()
-    if root == nil {
-        return res
-    }
-    var stack = [TreeNode]()
-    stack.append(root!)
+    var result = [Int]()
+    guard let root = root else { return result }
+    var stack = [root]
     while !stack.isEmpty {
-        let node = stack.popLast()!
-        res.append(node.val)
-        if node.right != nil {
-            stack.append(node.right!)
+        let current = stack.removeLast()
+        // 先右后左，这样出栈的时候才是左右顺序
+        if let node = current.right { // 右
+            stack.append(node)
         }
-        if node.left != nil {
-            stack.append(node.left!)
+        if let node = current.left { // 左
+            stack.append(node)
         }
+        result.append(current.val) // 中
     }
-    return res
+    return result
 }
-```
 
-> 迭代法中序遍历
-```swift
-func inorderTraversal(_ root: TreeNode?) -> [Int] {
-    var res = [Int]()
-    if root == nil {
-        return res
-    }
-    var stack = [TreeNode]()
-    var cur: TreeNode? = root
-    while cur != nil || !stack.isEmpty {
-        if cur != nil {
-            stack.append(cur!)
-            cur = cur!.left
-        } else {
-            cur = stack.popLast()
-            res.append(cur!.val)
-            cur = cur!.right
-        }
-    }
-    return res
-}
-```
-
-> 迭代法后序遍历
-```swift
+// 后序遍历迭代法
 func postorderTraversal(_ root: TreeNode?) -> [Int] {
-    var res = [Int]()
-    if root == nil {
-        return res
-    }
-    var stack = [TreeNode]()
-    stack.append(root!)
-    // res 存储 中 -> 右 -> 左
+    var result = [Int]()
+    guard let root = root else { return result }
+    var stack = [root]
     while !stack.isEmpty {
-        let node = stack.popLast()!
-        res.append(node.val)
-        if node.left != nil  {
-            stack.append(node.left!)
+        let current = stack.removeLast()
+        // 与前序相反，即中右左，最后结果还需反转才是后序
+        if let node = current.left { // 左
+            stack.append(node)
         }
-        if node.right != nil  {
-            stack.append(node.right!)
+        if let node = current.right { // 右
+            stack.append(node)
+        }
+        result.append(current.val) // 中
+    }
+    return result.reversed()
+}
+
+// 中序遍历迭代法
+func inorderTraversal(_ root: TreeNode?) -> [Int] {
+    var result = [Int]()
+    var stack = [TreeNode]()
+    var current: TreeNode! = root
+    while current != nil || !stack.isEmpty {
+        if current != nil { // 先访问到最左叶子
+            stack.append(current)
+            current = current.left // 左
+        } else {
+            current = stack.removeLast()
+            result.append(current.val) // 中
+            current = current.right // 右
         }
     }
-    // res 翻转
-    res.reverse()
-    return res
+    return result
 }
 ```