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题目描述:

方法一：递归

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

 方法二：迭代

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

 

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