15. DFS Postorder

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Input: root = [1, null, 2, 3]

Output: [3, 2, 1]

Explanation: The postorder traversal is left subtree (3), right subtree (2), then root (1).

Input: root = []

Output: []

Explanation: An empty tree results in an empty traversal.

Input: root = [1]

Output: [1]

Explanation: A single-node tree simply returns the node's value.

Time Complexity

O(n)

Space Complexity

O(h)