1382.js

/** ------------------------------------------------------------------------------
 * 
 * 1382. Balance a Binary Search Tree
 * Topics: BST
 * https://leetcode.com/problems/balance-a-binary-search-tree/description/
 * 
 ------------------------------------------------------------------------------ */
/**
 * 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 {TreeNode} root
 * @return {TreeNode}
 */ var balanceBST = function (root) {
  let arr = []
  function dfs(root) {
    if (root == null) return
    dfs(root.left)
    arr.push(root.val)
    dfs(root.right)
  }

  dfs(root)

  return sortedArrayToBST(arr)
}

var sortedArrayToBST = function (nums) {
  const fn = (nums, left, right) => {
    if (left >= right) return null
    const mid = Math.floor((left + right) / 2)
    return new TreeNode(nums[mid], fn(nums, left, mid), fn(nums, mid + 1, right))
  }

  return fn(nums, 0, nums.length)
}