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.
题目大意：基于一个二叉搜索树实现一个iterator，调用next()返回BST中下一个最小的数。
解题思路：我是在生成这个iterator的时候，预处理这棵树，以后所有的操作都是O(1)。
public class BSTIterator { private Stack<TreeNode> stack = new Stack<>(); public BSTIterator(TreeNode root) { init(root); } private void init(TreeNode node){ if(node==null){ return; } init(node.right); stack.push(node); init(node.left); } /** @return whether we have a next smallest number */ public boolean hasNext() { return !stack.isEmpty(); } /** @return the next smallest number */ public int next() { return stack.pop().val; }
Binary Search Tree Iterator——LeetCode
原文：http://www.cnblogs.com/aboutblank/p/4423095.html