LeetCode Binary Search Tree Iterator

原题链接在这里：https://leetcode.com/problems/binary-search-tree-iterator/

AC Java:

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

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

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

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

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

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