LeetCode 144.二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

 示例:

输入: [1,null,2,3]  
   1
    
     2
    /
   3 

输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if root is None:
            return []
        ans = []
        stack = [(0,root)]
        while stack:
            flag, node = stack.pop()
            if node is None:
                continue
            if flag == 0:              
                stack.append((0, node.right))
                stack.append((0, node.left))
                stack.append((1, node))                
            else:
                ans.append(node.val)
        return ans

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if root is None:
            return []
        stack = [root]
        ans = []
        while stack:
            node = stack.pop()
            ans.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return ans