4.3 Given a sorted (increasing order) array with unique integer elements, write an algorithm to create a binary search tree with minimal height.

4.3 sortedArrayToBST

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    
    TreeNode(int x) {
        val = x;
    }
}

public TreeNode sortedArrayToBST(int[] num) {
    if (num == null || num.length == 0) {
        return null;
    }
        
    return helper(num, 0, num.length - 1);
}
    
private TreeNode helper(int[] num, int start, int end) {
    if (start > end) {
        return null;
    }
        
    int mid = start + (end - start) / 2;
    TreeNode root = new TreeNode(num[mid]);
    root.left = helper(num, start, mid - 1);
    root.right = helper(num, mid + 1, end);
        
    return root;
}