[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.

Callingnext()Will return the next smallest number in the BST.

Note:next()AndhasNext()Shocould run in average O (1) time and uses O (h) memory, where h is the height of the tree.

This is a classic question. It is a non-recursive central order traversal. In fact, this question is equivalent to writing an iterator for sequential traversal in a binary tree. A stack needs to be built in to store the path to the leftmost leaf node at the beginning. In the traversal process, as long as the current node has the right child, it will enter the right child and save the path from this place to the leftmost leaf node in the current subtree.

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;}}