173-BSTIter.java

import java.util.*;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class BSTIterator_173 {
    private Stack<TreeNode> stack = new Stack<>();
    // 用栈来中序遍历
    // 构造函数
    public BSTIterator_173(TreeNode root) {
        while(root != null) {
            stack.push(root);
            root = root.left;
        }
    }
    
    /** @return the next smallest number */
    public int next() {
        TreeNode cur = stack.pop();
        int res = cur.val;
        cur = cur.right;
        while(cur!=null) {
            stack.push(cur);
            cur = cur.left;
        }
        return res;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */