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.

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))  # []

Approach

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

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

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

Hints

💡 Hint 1 — Base Case

What do you return when the tree is empty? There are no values to collect, so return an empty list.

if root is None:
    return []

💡 Hint 2 — Build the List

After collecting the left and right values recursively, put them together with the current node's value first.

left_vals  = get_values(root.left)
right_vals = get_values(root.right)
return [root.val] + left_vals + right_vals

✅ Solution

def get_values(root):
    if root is None:
        return []
    left_vals  = get_values(root.left)
    right_vals = get_values(root.right)
    return [root.val] + left_vals + right_vals

Why this works: Each node returns a list starting with itself, followed by everything in its left subtree, then everything in its right subtree. Leaf nodes return [leaf.val] + [] + [] = [leaf.val]. These lists get joined as the recursion unwinds.

Collect All Values

Exercise: Collect All Values

Your Task

Examples

Approach

Hints

Try it yourself

Code: Collect All Values