leetcode-144-二叉树的前序遍历

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        def helper(root):
            if not root:
                return
            res.append(root.val)
            helper(root.left)
            helper(root.right)
            
        helper(root)
        return res

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res,stack = [],[root]
        if not root:
            return res
        while stack:
            node = stack.pop()
            res.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return res