LeetCode 173: Binary Search Tree Iterator

Implement 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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    // Adopt inorder traversal
    Queue<TreeNode> queue;
    
    public BSTIterator(TreeNode root) {
        queue = new LinkedList<>();
        inorder(root);
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if (!queue.isEmpty())
            return true;
        else
            return false;
    }

    /** @return the next smallest number */
    public int next() {
        return queue.remove().val;
    }
    
    private void inorder(TreeNode root)
    {
        if (root == null)
            return;
            
        inorder(root.left);
        queue.add(root);
        inorder(root.right);
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */