Binary Search Tree Iterator

描述

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.

分析

考察非递归的中序遍历。这道题本质上是写一个二叉树的中序遍历的迭代器。内部设置一个栈，初始化的时候，存储从根节点到最左叶子节点的路径。在遍历的过程中，每次从栈中弹出一个元素，作为当前的返回结果，同时探测一下当前节点是否存在右孩子，如果有，则进入右孩子，并把从该右孩子到最左叶子节点的所有节点入栈。

代码

  1. // Binary Search Tree Iterator
  2. public class BSTIterator {
  3.     public BSTIterator(TreeNode root) {
  4.         stack = new Stack<>();
  5.         while (root != null) {
  6.             stack.push(root);
  7.             root = root.left;
  8.         }
  9.     }
  11.     /** @return whether we have a next smallest number */
  12.     public boolean hasNext() {
  13.         return !stack.isEmpty();
  14.     }
  16.     /** @return the next smallest number */
  17.     public int next() {
  18.         final TreeNode node = stack.pop();
  19.         if (node.right != null) {
  20.             TreeNode p = node.right;
  21.             while (p != null) {
  22.                 stack.push(p);
  23.                 p = p.left;
  24.             }
  26.         }
  27.         return node.val;
  28.     }
  29.     private Stack<TreeNode> stack;
  30. }

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/java/binary-tree/traversal/binary-search-tree-iterator.html