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144_BinaryTreePreorderTraversal.py
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144_BinaryTreePreorderTraversal.py
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# Definition for a binary tree node.
class TreeNode(object):
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution(object):
def preorderTraversal(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
# Preorder traversal <Root Left Right>
# Recursive way
"""
def helper(root, result):
if root:
result.append(root.val)
helper(root.left, result)
helper(root.right, result)
result = []
helper(root, result)
return result
"""
# Iteratvie way O(n), O(n)
result, stack = [], []
while True:
while root:
# append the root value first to the result
result.append(root.val)
# Hold on to the right value
stack.append(root.right)
# Traverse to the left leaf
root = root.left
if not stack:
return result
# Finsh the left branches and the root, back to right leaves
root = stack.pop()
if __name__ == '__main__':
root = TreeNode(1)
root.right = TreeNode(2)
root.right.left = TreeNode(3)
print(Solution().preorderTraversal(root))