LeetCode/problems/binary-search-tree-iterator.java

import java.util.ArrayDeque;

/* *
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class BSTIterator {
    private enum Position {
        Todo, Done
    }

    private class Entry {
        public TreeNode node;
        public Position position;

        public Entry(TreeNode node, Position position) {
            this.node = node;
            this.position = position;
        }

        public void expand(ArrayDeque<Entry> stack) {
            if (node.right != null) {
                stack.addLast(new Entry(node.right, Position.Todo));
            }

            this.position = Position.Done;
            stack.addLast(this);

            if (node.left != null) {
                stack.addLast(new Entry(node.left, Position.Todo));
            }
        }
    }

    private ArrayDeque<Entry> stack = new ArrayDeque<>();
    public BSTIterator(TreeNode root) {
        stack.addLast(new Entry(root, Position.Todo));
    }

    public int next() {
        for (
            Entry last = stack.getLast();
            last != null && last.position == Position.Todo;
            last = stack.getLast()
        ) {
            stack.removeLast().expand(stack);
        }

        return stack.removeLast().node.val;
    }

    public boolean hasNext() {
        Entry last = stack.peekLast();
        return last != null && (last.position != Position.Todo || last.node != null);
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */