add 589. N-ary Tree Preorder Traversal

Author: hungwei0331

Explore/N-ary Tree/589. N-ary Tree Preorder Traversal/MyPlayground.playground/Contents.swift

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+/*
+ Given the root of an n-ary tree, return the preorder traversal of its nodes' values.
+ Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)
+ 
+ Example 1:
+ Input: root = [1,null,3,2,4,null,5,6]
+ Output: [1,3,5,6,2,4]
+ 
+ Example 2:
+ Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
+ Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]
+ */
+
+public class Node {
+    public var val: Int
+    public var children: [Node]
+    public init(_ val: Int) {
+        self.val = val
+        self.children = []
+    }
+}
+
+// TC: O(n), where N is the number of nodes
+// SC: O(n), we could keep up to the entire tree
+// Recursive
+class Solution {
+    func preorder(_ root: Node?) -> [Int] {
+        var result = [Int]()
+        
+        func traverse(_ node: Node?) {
+            guard let node = node else { return }
+            result.append(node.val)
+            for child in node.children {
+                traverse(child)
+            }
+        }
+        
+        traverse(root)
+        return result
+    }
+}
+
+// Iteratively
+class Solution1 {
+    func preorder(_ root: Node?) -> [Int] {
+        guard let root = root else { return [] }
+        var result = [Int]()
+        var stack = [root]
+        
+        while !stack.isEmpty {
+            let node = stack.removeLast()
+            result.append(node.val)
+            stack.append(contentsOf: node.children.reversed())
+        }
+        
+        return result
+    }
+}
+
+

Explore/N-ary Tree/589. N-ary Tree Preorder Traversal/MyPlayground.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='7.0' target-platform='ios' swift-version='6' buildActiveScheme='true' importAppTypes='true'/>