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.




// Binary Search Tree Iterator
public class BSTIterator {
    public BSTIterator(TreeNode root) {
        stack = new Stack<>();
        while (root != null) {
            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() {
        final TreeNode node = stack.pop();
        if (node.right != null) {
            TreeNode p = node.right;
            while (p != null) {
                p = p.left;

        return node.val;
    private Stack<TreeNode> stack;

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