LeetCode 173. Binary Search Tree Iterator（二叉搜索树迭代器）

原题网址：https://leetcode.com/problems/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.

方法：用栈保存状态。

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class BSTIterator {    private Stack<TreeNode> stack = new Stack<>();    public BSTIterator(TreeNode root) {        TreeNode current = root;        while (current != null) {            stack.push(current);            current = current.left;        }    }    /** @return whether we have a next smallest number */    public boolean hasNext() {        return !stack.isEmpty();    }    /** @return the next smallest number */    public int next() {        TreeNode next = stack.pop();        TreeNode current = next.right;        while (current != null) {            stack.push(current);            current = current.left;        }        return next.val;    }}/** * Your BSTIterator will be called like this: * BSTIterator i = new BSTIterator(root); * while (i.hasNext()) v[f()] = i.next(); */