package com.hit.basmath.learn.binary_search_tree;

import com.hit.common.TreeNode;

import java.util.Stack;

/**
 * 173. Binary Search Tree Iterator
 * <p>
 * Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.
 * <p>
 * Calling next() will return the next smallest number in the BST.
 * <p>
 * Example:
 * <p>
 * BSTIterator iterator = new BSTIterator(root);
 * iterator.next();    // return 3
 * iterator.next();    // return 7
 * iterator.hasNext(); // return true
 * iterator.next();    // return 9
 * iterator.hasNext(); // return true
 * iterator.next();    // return 15
 * iterator.hasNext(); // return true
 * iterator.next();    // return 20
 * iterator.hasNext(); // return false
 * <p>
 * Note:
 * <p>
 * 1. next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
 * 2. You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.
 */
public class _173 {
    class BSTIterator {

        private Stack<TreeNode> stack = new Stack<TreeNode>();

        public BSTIterator(TreeNode root) {
            pushAll(root);
        }

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

        /**
         * @return the next smallest number
         */
        public int next() {
            TreeNode tmpNode = stack.pop();
            pushAll(tmpNode.right);
            return tmpNode.val;
        }

        private void pushAll(TreeNode node) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }
        }
    }
}