You are given the root of a binary tree. Your task is to return the inorder traversal of the tree.

Input: root = [1,null,2,3]
Output: [1,3,2]
Explanation: The nodes are visited in Left → Root → Right order.

Input: root = [4,2,6,1,3,5,7]
Output: [1,2,3,4,5,6,7]
Explanation: The inorder traversal visits the left subtree, root, and then the right subtree.
Input: root = []
Output: []
Explanation: The tree is empty.
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