添加257. 二叉树的所有路径 Swift 版本

problems/0257.二叉树的所有路径.md

@@ -581,6 +581,97 @@ var binaryTreePaths = function(root) {
 };  
 ```
 
+Swift:
+
+递归法：
+
+```swift
+func binaryTreePaths(_ root: TreeNode?) -> [String] {
+        var result = Array<String>()
+        var path = Array<Int>()
+        guard let root = root else {
+            return result
+        }
+        traversal(root, &path, &result)
+        return result
+    }
+    
+    func traversal(_ cur: TreeNode, _ path: inout Array<Int>, _ result: inout Array<String>) {
+        path.append(cur.val)
+        // 递归终止条件：到达叶子节点
+        if cur.left == nil && cur.right == nil {
+            var pathString = ""
+            // 处理 path 前面的元素
+            for i in 0..<path.count - 1 {
+                let value = path[i]
+                pathString += String(value)
+                pathString += "->"
+            }
+            // 处理 path 最后一个元素
+            if let lastValue = path.last {
+                pathString += String(lastValue)
+            }
+            // 将路径存到数组中
+            result.append(pathString)
+            return
+        }
+        
+        // 空节点不进入递归
+        if let leftNode = cur.left {
+            traversal(leftNode, &path, &result)
+            // 回溯
+            path.removeLast()
+        }
+        // 空节点不进入递归
+        if let rightNode = cur.right {
+            traversal(rightNode, &path, &result)
+            // 回溯
+            path.removeLast()
+        }
+    }
+```
+
+迭代法：
+
+```swift
+func binaryTreePaths(_ root: TreeNode?) -> [String] {
+        // 保存树的遍历节点
+        var treeStack = [TreeNode]()
+        // 保存遍历路径的节点
+        var pathStack = [String]()
+        // 保存最终路径集合
+        var result = [String]()
+        guard let root = root else {
+            return result
+        }
+        treeStack.append(root)
+        pathStack.append(String(root.val))
+        while !treeStack.isEmpty {
+            let node = treeStack.removeLast()
+            let path = pathStack.removeLast()
+            
+            // 遇到叶子节点
+            if node.left == nil && node.right == nil {
+                result.append(path)
+            }
+            
+            if let rightNode = node.right {
+                treeStack.append(rightNode)
+                let tmp = path + "->" + String(rightNode.val)
+                pathStack.append(tmp)
+            }
+            
+            if let leftNode = node.left {
+                treeStack.append(leftNode)
+                let tmp = path + "->" + String(leftNode.val)
+                pathStack.append(tmp)
+            }
+        }
+        
+        return result
+    }
+```
+
 
 
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