[LeetCode] 589. N-ary Tree Preorder Traversal_Easy

Given an n-ary tree, return the preorder traversal of its nodes' values.

For example, given a 3-ary tree:

Return its preorder traversal as: [1,3,5,6,2,4].

这个题目思路就是跟LeetCode questions conlusion_InOrder, PreOrder, PostOrder traversal类似, recursive 和iterable都可以.

04/17/2019 update: 加第三种方法， 利用divide and conquer

1. Recursively

class Solution:
    def nary_Preorder(self, root):
        def helper(root):
            if not root: return
            ans.append(root.val)
            for each in root.children:
                helper(each)
        ans = []
        helper(root)
        return ans

2. Iterable

class Solution:
    def nary_preOrder(self, root):
        if not root: return []
        stack, ans = [root], []
        while stack:
            node = stack.pop()
            ans.append(node.val)
            for each in node.children[::-1]:
                stack.append(each)
        return ans

3. Divide and conquer

# Divide and conquer
class Solution:
    def preOrder(self, root):
        if not root: return []
        temp = []
        for each in root.children:
            temp += self.preOrder(each)
        return [root.val] + temp