[LeetCode] Binary Search Tree Iterator，leetcodeiterator

[LeetCode] Binary Search Tree Iterator，leetcodeiterator

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.

這是一道很經典的題目，考的非遞迴的中序遍曆。其實這道題等價於寫一個二叉樹中序遍曆的迭代器。需要內建一個棧，一開始先儲存到最左葉子節點的路徑。在遍曆的過程中，只要當前節點存在右孩子，則進入右孩子，存除從此處開始到當前子樹裡最左葉子節點的路徑。

public class BSTIterator {Stack<TreeNode> stack;public BSTIterator(TreeNode root) {stack = new Stack<TreeNode>();while (root != null) {stack.push(root);root = root.left;}}/** @return whether we have a next smallest number */public boolean hasNext() {return !stack.isEmpty();}/** @return the next smallest number */public int next() {TreeNode node = stack.pop();int ret = node.val;if (node.right != null) {node = node.right;while (node != null) {stack.push(node);node = node.left;}}return ret;}}