Post Order Traversal Iterator

import java.util.Stack;

class PostorderIterator {
    private Stack<Node> stack = new Stack<>();
    private Node prevProcessed = null;

    public PostorderIterator(Node root) {
        pushLeftSubtree(root);
    }

    private void pushLeftSubtree(Node node) {
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
    }

    public int next() {
        if (!hasNext()) {
            return -1;
        }

        while (!stack.isEmpty()) {
            Node curr = stack.peek();
            // If there's no right child OR right child has already been processed
            if (curr.right == null || curr.right == prevProcessed) {
                stack.pop();
                prevProcessed = curr;
                return curr.val; // Corrected return statement
            }

            // Process right subtree
            pushLeftSubtree(curr.right);
        }

        return -1; // Should never reach here if hasNext() is checked before calling next()
    }

    public boolean hasNext() {
        return !stack.isEmpty();
    }
}

class Node {
    int val;
    Node left, right;

    public Node(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}