如何将char数组转换为BST？

class TreeNode {
    char val;
    TreeNode left;
    TreeNode right;

    TreeNode(char val) {
        this.val = val;
    }
}

public class CharArrayToBST {
    public static TreeNode charArrayToBST(char[] arr) {
        if (arr == null || arr.length == 0) {
            return null;
        }
        TreeNode root = null;
        for (char c : arr) {
            root = insert(root, c);
        }
        return root;
    }

    private static TreeNode insert(TreeNode root, char val) {
        if (root == null) {
            return new TreeNode(val);
        }
        if (val < root.val) {
            root.left = insert(root.left, val);
        } else if (val > root.val) {
            root.right = insert(root.right, val);
        }
        return root;
    }

    public static void main(String[] args) {
        char[] arr = {'b', 'a', 'c', 'd'};
        TreeNode root = charArrayToBST(arr);
        // 打印BST的中序遍历结果，即按顺序输出字符
        inorderTraversal(root);
    }

    private static void inorderTraversal(TreeNode root) {
        if (root != null) {
            inorderTraversal(root.left);
            System.out.print(root.val + " ");
            inorderTraversal(root.right);
        }
    }
}