Binary Search Tree Iterator

mplement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Use a stack to store all left child of the tree (pushNode()). When calling next() method, pop() the tail of the stack. If the node has a right child, store all left children of the right child (pushNode(right)).

public class BSTIterator {
    
    private Stack stack;

    public BSTIterator(TreeNode root) {
        stack = new Stack ();
        pushNode(root);
        
    }
    private void pushNode(TreeNode root) {
        while (root != null) {
            stack.push(root);
            root = root.left;
        }
    }
    

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        if (stack.isEmpty()) {
            throw new Error("All nodes have been visited");
        }
        TreeNode curr = stack.pop();
        if(curr.right != null) {
            pushNode(curr.right);
        }
        return curr.val;
    }
}