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 BinarySearchTreeIterator {
    Stack<TreeNode> stack;

    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        while(root != null) {
            stack.push(root);
            root = root.left;
        }
    }

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

    /** @return the next smallest number */
    public int next() {
        if (!hasNext()) return -1;
        TreeNode node = stack.pop();
        if (node.right != null) {
            TreeNode cur = node.right;
            while (cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
        }
        return node.val;
    }
}

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