添加 404. 左叶子之和 Swift 版本

problems/0404.左叶子之和.md

@@ -373,6 +373,54 @@ var sumOfLeftLeaves = function(root) {
 ```
 
 
+## Swift 
+
+**递归法** 
+```swift
+func sumOfLeftLeaves(_ root: TreeNode?) -> Int {
+    guard let root = root else {
+        return 0
+    }
+
+    let leftValue = sumOfLeftLeaves(root.left)
+    let rightValue = sumOfLeftLeaves(root.right)
+
+    var midValue: Int = 0
+    if root.left != nil && root.left?.left == nil && root.left?.right == nil {
+        midValue = root.left!.val
+    }
+
+    let sum = midValue + leftValue + rightValue
+    return sum
+}
+```
+**迭代法**
+```swift
+func sumOfLeftLeaves(_ root: TreeNode?) -> Int {
+    guard let root = root else {
+        return 0
+    }
+
+    var stack = Array<TreeNode>()
+    stack.append(root)
+    var sum = 0
+
+    while !stack.isEmpty {
+        let lastNode = stack.removeLast()
+
+        if lastNode.left != nil && lastNode.left?.left == nil && lastNode.left?.right == nil {
+            sum += lastNode.left!.val
+        }
+        if let right = lastNode.right {
+            stack.append(right)
+        }
+        if let left = lastNode.left {
+            stack.append(left)
+        }
+    }
+    return sum
+}
+```