Java for LeetCode 173 Binary Search Tree Iterator

標籤：

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.

解題思路：

其實就是按照中序遍曆的順序出列，有兩種實現思路：

一、構造BST的時候直接預先處理下，構造出一條按照中序遍曆的list，每次讀取list的元素即可。

二、不進行預先處理，按照中序遍曆的思路使用Stack動態進行處理

這裡我們實現思路二：

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

 

Java for LeetCode 173 Binary Search Tree Iterator