https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/

constructBinaryTreefromInorderandPostorderTraversal.js

/**
 * 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;
    
    const idx = {};
    inorder.forEach((v, i) => idx[v] = i);
    
    const build = (l, r) => {
        if(l > r) return null;
        
        const node = new TreeNode(postorder.pop());
        if(l === r) return node;
        
        const index = idx[node.val];
        node.right = build(index+1, r);
        node.left  = build(l, index-1);
        
        return node;
    };
    
    return build(0, inorder.length-1);
};