/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    //栈用于迭代
    Stack<TreeNode> stack;
    TreeNode cur;
    //记录当前节点
    public BSTIterator(TreeNode root) {
        //初始化两个值
        stack = new Stack<TreeNode>();
        cur = root;
    }
    
    public int next() {
        //返回一个节点
        while(cur != null){
            stack.push(cur);
            cur = cur.left;
        }
        cur = stack.pop();
        int val = cur.val;
        cur = cur.right;
        return val;
    }
    
    public boolean hasNext() {
        //判断当前是否还有节点可以迭代，就是判断栈中是否为空或者当前节点是否还能接着将节点加入栈
        return !stack.isEmpty() || cur != null;
    }
}

/**
 * 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();
 */