144. Binary Tree Preorder Traversal¶

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# @lc app=leetcode id=144 lang=python3
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# [144] Binary Tree Preorder Traversal
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# https://leetcode.com/problems/binary-tree-preorder-traversal/description/
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# algorithms
# Easy (66.96%)
# Likes:    7805
# Dislikes: 203
# Total Accepted:    1.5M
# Total Submissions: 2.2M
# Testcase Example:  '[1,null,2,3]'
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# Given the root of a binary tree, return the preorder traversal of its nodes'
# values.
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# Example 1:
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# Input: root = [1,null,2,3]
# Output: [1,2,3]
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# Example 2:
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# Input: root = []
# Output: []
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# Example 3:
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# Input: root = [1]
# Output: [1]
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# Constraints:
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# The number of nodes in the tree is in the range [0, 100].
# -100 <= Node.val <= 100
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# Follow up: Recursive solution is trivial, could you do it iteratively?
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# @lc code=start
# Definition for a binary tree node.
from typing import Optional, List

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]:
        o = []
        traversal(root, o)
        return o

def traversal(n, o):
    if n is None:
        return
    o.append(n.val)
    traversal(n.left, o)
    traversal(n.right, o)

# @lc code=end