[LeetCode]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.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

题解：使用Stack，由于BST，中序遍历就是从小到大。

code:

public class BSTIterator {private Stack<TreeNode> stack = null;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() {if(hasNext()){int ret = stack.peek().val;TreeNode cur = stack.pop();if(cur.right!=null){cur = cur.right;while(cur!=null){stack.push(cur);cur = cur.left;}}return ret;}return -1;}}