LeetCode 173. Binary Search Tree Iterator（二叉搜索树迭代器）

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本文介绍了一种使用栈来实现二叉搜索树迭代器的方法，该方法可以在平均O(1)的时间复杂度内完成next()和hasNext()操作，并且仅使用O(h)的空间，其中h为树的高度。

原题网址：https://leetcode.com/problems/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.

方法：用栈保存状态。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    private Stack<TreeNode> stack = new Stack<>();

    public BSTIterator(TreeNode root) {
        TreeNode current = root;
        while (current != null) {
            stack.push(current);
            current = current.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode next = stack.pop();
        TreeNode current = next.right;
        while (current != null) {
            stack.push(current);
            current = current.left;
        }
        return next.val;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */