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144. Binary Tree Preorder Traversal

https://leetcode.com/problems/binary-tree-preorder-traversal/description/

Description

Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]

   1
    \
     2
    /
   3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        from collections import deque
        ret = []
        q = deque([root])
        while len(q):
            cur_node = q.popleft()
            if cur_node != None:
                ret.append(cur_node.val)
                q.appendleft(cur_node.right)
                q.appendleft(cur_node.left)
        return ret