给定两个整数数组 inorder 和 postorder ，其中 inorder 是二叉树的中序遍历， postorder 是同一棵树的后序遍历，请你构造并返回这颗 二叉树 。

示例 1:

输入：inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
输出：[3,9,20,null,null,15,7]

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {number[]} inorder
 * @param {number[]} postorder
 * @return {TreeNode}
 */
var buildTree = function(inorder, postorder) {
  if(!postorder.length) return null
  let rootIndex = postorder.length - 1
  let rootVal = postorder[rootIndex]
  let root = new TreeNode(rootVal)

  let centerl = inorder.slice(0, inorder.indexOf(rootVal))
  let centerr = inorder.slice(inorder.indexOf(rootVal) + 1)

  let afterl = postorder.slice(0, inorder.indexOf(rootVal))
  let afterr = postorder.slice(inorder.indexOf(rootVal), rootIndex)

  root.left = buildTree(centerl, afterl)
  root.right = buildTree(centerr, afterr)

  

  return root
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal