109.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {TreeNode}
 */
var sortedListToBST = function(head) {
  const sortedArrayToBST = function(nums) {
    if (nums.length === 0) return null;
    if (nums.length === 1) return new TreeNode(nums[0]);

    const midIndex = Number.parseInt(nums.length / 2);
    const returnValue = new TreeNode(nums[midIndex]);
    returnValue.left = sortedArrayToBST(nums.slice(0, midIndex));
    returnValue.right = sortedArrayToBST(nums.slice(midIndex + 1, nums.length));

    return returnValue;
  };

  const nums = [];
  while(head) {
    nums.push(head.val);
    head = head.next;
  }
  return sortedArrayToBST(nums);
};