144. 二叉树的前序遍历

给你二叉树的根节点 root ，返回它节点值的 前序 遍历。
进阶：递归算法很简单，你可以通过迭代算法完成吗？

Python:
1.递归：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        def preOrder(root):
            if not root:
                return
            else:
                res.append(root.val)
                preOrder(root.left)
                preOrder(root.right)
        preOrder(root)
        return res

2.非递归

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        q = root
        queue = []
        while q or queue:
            if q:
                res.append(q.val)
                queue.append(q)
                q = q.left
            else:
                temp = queue.pop()
                if temp.right:
                    q = temp.right
        return res