LeetCode 108. Convert Sorted Array to Binary Search Tree (javascript)

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Example 1:

Input: nums = [-10,-3,0,5,9]
Output: [0,-3,9,-10,null,5]
Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Example 2:

Input: nums = [1,3]
Output: [3,1]
Explanation: [1,3] and [3,1] are both a height-balanced BSTs.

Constraints:

1 <= nums.length <= 10⁴
-10⁴ <= nums[i] <= 10⁴
nums is sorted in a strictly increasing order.

Idea:

Recursion + Divide and Conquer

Time complexity: O(n)
Space complexity: O(logn)

Solution:

/**
 * 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[]} nums
 * @return {TreeNode}
 */
var sortedArrayToBST = function(nums) {
    return createBST(nums, 0, nums.length - 1);
};

function createBST(nums, l, r) {
    // exit recursion condition
    if (l > r) return null;
    
    let mid = l + Math.floor((r - l) / 2);
    let root = new TreeNode(nums[mid]);
    root.left = createBST(nums, l, mid - 1);
    root.right = createBST(nums, mid + 1, r);
    return root;
}