Exercise: Collect All Values

Your Task

Write a function get_values(root) that returns a list of all node values in the tree, visited in preorder order: root first, then left subtree, then right subtree.

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Examples

#       1
#      / \
#     2   3
#    / \
#   4   5

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)

print(get_values(root))  # [1, 2, 4, 5, 3]

#   7
#    \
#     9

root = TreeNode(7)
root.right = TreeNode(9)

print(get_values(root))  # [7, 9]

print(get_values(None))  # []

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Approach

This is almost identical to the inorder example from the intro — just change the order:

• Preorder: [root.val] + left values + right values

1. Base case: if root is None, return an empty list []
2. Collect all values from the left subtree
3. Collect all values from the right subtree
4. Return [root.val] + left_values + right_values

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Hints