1008. Construct Binary Search Tree from Preorder Traversal.js

// Return the root node of a binary search tree that matches the given preorder traversal.
// (Recall that a binary search tree is a binary tree where for every node, any descendant of node.left has a value < node.val, and any descendant of node.right has a value > node.val.  Also recall that a preorder traversal displays the value of the node first, then traverses node.left, then traverses node.right.)
//
// Example 1:
//
// Input: [8,5,1,7,10,12]
// Output: [8,5,10,1,7,null,12]
//
//       8
//      / \
//     5   10
//    / \    \
//   1   7    12
//
// Note:
//
// 1 <= preorder.length <= 100
// The values of preorder are distinct.

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number[]} preorder
 * @return {TreeNode}
 */
const bstFromPreorder = (preorder) => {
  if (preorder == null || preorder.length === 0) return null;
  const [mid, ...rest] = preorder;
  const root = new TreeNode(mid);
  root.left = bstFromPreorder(rest.filter(n => n < mid));
  root.right = bstFromPreorder(rest.filter(n => n > mid));
  return root;
};