Construct binary tree from preorder and postorder

// Construct binary tree from preorder and postorder traversal - medium - 02/09/2023

class Solution {
    private HashMap<Integer, Integer> postorderMap= new HashMap<>();
    public TreeNode constructFromPrePost(int[] preorder, int[] postorder) 
    {
        for(int i= 0; i< postorder.length; i++)
            postorderMap.put(postorder[i], i);
        
        return constructBT(preorder, 0, preorder.length - 1, 0, postorder.length - 1);
    }
    public TreeNode constructBT(int[] preorder, int preorderStart, int preorderEnd, int postorderStart, int postorderEnd) 
    { 
        if(preorderStart > preorderEnd)
            return null;
        
        int rootData= preorder[preorderStart];
 
        if(preorderStart == preorderEnd)
            return new TreeNode(rootData, null, null);
        
        int  LeftrootIndex= postorderMap.get(preorder[preorderStart + 1]);
        
        int leftSubtreeSize= LeftrootIndex - postorderStart + 1; 
            
        TreeNode left= constructBT(preorder, preorderStart+ 1, preorderStart + leftSubtreeSize, postorderStart, LeftrootIndex);
        TreeNode right= constructBT(preorder, preorderStart + leftSubtreeSize + 1, preorderEnd, LeftrootIndex + 1, preorderEnd - 1);        

        
        TreeNode node= new TreeNode(rootData, left, right);
        
        return node; 
    }
}

// Optimized 

int preIndex = 0, posIndex = 0;
    public TreeNode constructFromPrePost(int[]pre, int[]post) {
        TreeNode root = new TreeNode(pre[preIndex++]);
        if (root.val != post[posIndex])
            root.left = constructFromPrePost(pre, post);
        if (root.val != post[posIndex])
            root.right = constructFromPrePost(pre, post);
        posIndex++;
        return root;
    }