Untitled

public class Solution {
    private int index = 0;

    public TreeNode bstFromPreorder(int[] preorder) {
        return construct(preorder, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    private TreeNode construct(int[] preorder, int lowerBound, int upperBound) {
        if (index >= preorder.length) return null;

        int val = preorder[index];
        if (val < lowerBound || val > upperBound) return null;

        index++; // Move to the next element
        TreeNode node = new TreeNode(val);
        node.left = construct(preorder, lowerBound, val); // Left subtree
        node.right = construct(preorder, val, upperBound); // Right subtree
        return node;
    }
}