Sorted Array to BST Solution

For a balanced tree, pick the middle element as root. This guarantees roughly equal elements on each side:

function sortedArrayToBST(nums):
    function build(left, right):
        if left > right:
            return null
        mid = floor((left + right) / 2)
        node = TreeNode(nums[mid])
        node.left = build(left, mid - 1)
        node.right = build(mid + 1, right)
        return node
    return build(0, nums.length - 1)

You're using divide and conquer. The middle element becomes the root, left half becomes left subtree, right half becomes right subtree.

Time: $O(n)$. Each element is visited once.

Space: $O(\log n)$ for recursion on balanced tree.

This technique also appears in building segment trees.