Reconstruct a binary tree from preorder traversal with mark

ConstructFromPreOrderTraversal.java

import java.util.List;

/*
      Problem: Given pre order traversal data of a binary tree where null markers are used when a node is empty
      construct binary tree
      {"H","B","F",null,null,"E","A",null,null,null,"C",null,"D",null,"G","I",null,null,null}
                     H
            / \
           /   \
          /     \
         /       \
        /         \
       /           \
      /             \
     /               \
     B               C
    / \               \
   /   \               \
  /     \               \
 /       \               \
 F       E               D
        /                 \
       /                   \
       A                   G
                          /
                          I
      Solution: In preorder traversal first comes root then left tree followed by its left tree until we hit null
      then right child. we can use this to make recursive calls like this
   */
public class ConstructFromPreOrderTraversal {
    private Integer subtreeIdx;
  
    public BinaryTreeNode<String> reconstructPreorder(List<String> preOrder) {
        subtreeIdx = 0;
        return reconstructPreorderHelper(preOrder);
    }

    public BinaryTreeNode<String> reconstructPreorderHelper(List<String> preOrder) {
        String subTreeKey = preOrder.get(subtreeIdx);
        subtreeIdx++;
        //Found null marker so return
        if(subTreeKey == null)
            return null;
        // Create left child
        BinaryTreeNode leftChild = reconstructPreorderHelper(preOrder);
        //Create right child
        BinaryTreeNode rightChild = reconstructPreorderHelper(preOrder);
        return new BinaryTreeNode<String>(subTreeKey,leftChild,rightChild);
    }
}