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Given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree, construct and return the binary tree.
Example 1
Input: inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
Output: [3,9,20,null,null,15,7]
Explanation: The last postorder value, 3, is the root. In the inorder list, 9 sits left of 3 (the left subtree) and [15,20,7] sits right of it. Applying the same rule to the right part makes 20 its root, with 15 on the left and 7 on the right.
Example 2
Input: inorder = [-1], postorder = [-1]
Output: [-1]
Explanation: Both traversals contain only -1, so the tree is that single node.
Example 3
Input: inorder = [2,1], postorder = [2,1]
Output: [1,2]
Explanation: The last postorder value, 1, is the root. In inorder, 2 comes before 1, so 2 is 1's left child.
Postorder visits the root last, so read postorder from the back to get roots. Each root splits inorder into left/right subtrees. Because we consume postorder in reverse (root, then right subtree, then left), build the right child before the left.
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