86题 Binary Search Tree Iterator

最新推荐文章于 2025-12-14 11:20:37 发布

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#队列 #二叉树 #算法

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本文介绍如何设计一个二叉搜索树迭代器，满足O(1)平均时间复杂度的next()和hasNext()操作，用于在升序遍历过程中获取下一个最小元素。通过队列实现高效访问，适用于数据结构与算法学习者。

Binary Search Tree Iterator

Description
Design an iterator over a binary search tree with the following rules:
Next() returns the next smallest element in the BST.

Elements are visited in ascending order (i.e. an in-order traversal)
next() and queries run in hasNext()O(1)O(1) time in average

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 * Example of iterate a tree:
 * BSTIterator iterator = new BSTIterator(root);
 * while (iterator.hasNext()) {
 *    TreeNode node = iterator.next();
 *    do something for node
 * } 
 */


public class BSTIterator {
    /**
    * @param root: The root of binary tree.
    */
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    public BSTIterator(TreeNode root) {
        // do intialization if necessary
        Iterator(root) ;

    }
    public void Iterator(TreeNode root){
        if (root == null){
            return   ;
        }
        Iterator(root.left) ;
        queue.offer(root) ;
        Iterator(root.right) ;
        
    }

    /**
     * @return: True if there has next node, or false
     */
    public boolean hasNext() {
        // write your code here
        return !queue.isEmpty() ;
    }

    /**
     * @return: return next node
     */
    public TreeNode next() {
        // write your code here
        return queue.poll() ;
    }
}