108. Convert Sorted Array to Binary Search Tree

Problem

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number[]} nums
 * @return {TreeNode}
 */
var sortedArrayToBST = function(nums) {
  return helper(nums, 0, nums.length - 1);
};

var helper = function (nums, ll, rr) {
  if (ll > rr) return null;
  var mid = Math.ceil((ll + rr) / 2);
  var root = new TreeNode(nums[mid]);
  root.left = helper(nums, ll, mid - 1);
  root.right = helper(nums, mid + 1, rr);
  return root;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).