Postorder Traversal

Return the values of the tree in postorder: visit the left subtree, then the right subtree, then the node itself.

The tests write a tree as a level-order list (row by row, left to right) with None for a missing child: [3, 9, 20, None, None, 15, 7] is a root 3 with children 9 and 20, where 20 has children 15 and 7.

For [1, None, 2, 3] the answer is [3, 2, 1].

Examples

Example 1

Input:

postorder_traversal(root)  # root = [1, None, 2, 3]

Output:

[3, 2, 1]

Example 2

Input:

postorder_traversal(root)  # root = [1, 2, 3, 4, 5]

Output:

[4, 5, 2, 3, 1]

Constraints

• 0 <= number of nodes <= 100000
• The tree is not deeper than 800 levels.
• -10**9 <= val <= 10**9