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						[1/9] 144. Binary Tree Preorder Traversal
본문
Given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1:

Input: root = [1,null,2,3] Output: [1,2,3]
Example 2:
Input: root = [] Output: []
Example 3:
Input: root = [1] Output: [1]
Constraints:
- The number of nodes in the tree is in the range [0, 100].
- -100 <= Node.val <= 100
Follow up: Recursive solution is trivial, could you do it iteratively?
Accepted
1.2M
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# 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: Optional[TreeNode]) -> List[int]:
        res = []
        def dfs(node):
            if not node: return
            res.append(node.val)
            dfs(node.left)
            dfs(node.right)
        
        dfs(root)
        return res 
								 
							








