Serialize Binary Tree

Problem

Serialize a binary tree into a preorder list with explicit None markers.

tree is a LeetCode-style level-order list with None marking missing nodes. Visit the root, then the left subtree, then the right subtree, appending None whenever a child is missing. An empty tree serializes to [].

Return the preorder serialization.

Hint

Push the right child before the left so the left subtree is processed first.

Test cases

case 1

in: [[1,2,3]]

expected: [1,2,null,null,3,null,null]

case 2

in: [[1]]

expected: [1,null,null]

case 3

in: [[]]

expected: []

case 4

in: [[1,null,2]]

expected: [1,null,2,null,null]

Starter codePython

def serialize_tree(tree):
    # Your code here
    pass

4 test cases · run your code against them in the editor