[刷题]Implement Iterator of Binary Search Tree

最新推荐文章于 2025-02-03 15:37:10 发布

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本文介绍如何实现一个用于遍历二叉搜索树的迭代器，包括中序遍历的实现方法，以及如何判断是否有下一个节点以及获取下一个节点。

[LintCode]Implement Iterator of Binary Search Tree

/**
 * 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:
 * Solution iterator = new Solution(root);
 * while (iterator.hasNext()) {
 *    TreeNode node = iterator.next();
 *    do something for node
 * } 
 */
public class Solution {
    //@param root: The root of binary tree.
    ArrayList<TreeNode> list;
    int index = 0; //指向下一个元素
    public Solution(TreeNode root) {
        // 2015-4-22 比较笨的方法需要O(n)的额外空间
        list = inorder(root);
    }
    
    private ArrayList<TreeNode> inorder(TreeNode root) {
        ArrayList<TreeNode> rst = new ArrayList<>();
        if (root == null) {
            return rst;
        }
        
        ArrayList<TreeNode> left = inorder(root.left);
        ArrayList<TreeNode> right = inorder(root.right);
        
        rst.addAll(left);
        rst.add(root);
        rst.addAll(right);
        return rst;
    }

    //@return: True if there has next node, or false
    public boolean hasNext() {
        int len = list.size();
        return index < len;
    }
    
    //@return: return next node
    public TreeNode next() {
        return list.get(index++);
    }
}