Binary Search Tree Iterator——LeetCode

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.

题目大意：基于一个二叉搜索树实现一个iterator，调用next()返回BST中下一个最小的数。

解题思路：我是在生成这个iterator的时候，预处理这棵树，以后所有的操作都是O(1)。

public class BSTIterator {

    private Stack<TreeNode> stack = new Stack<>();

    public BSTIterator(TreeNode root) {
        init(root);
    }

    private void init(TreeNode node){
        if(node==null){
            return;
        }
        init(node.right);
        stack.push(node);
        init(node.left);
    }

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

    /** @return the next smallest number */
    public int next() {
        return stack.pop().val;
    }