思路:

• 要求二叉搜索树的下一个最小值
• 就是求二叉搜索树的中序遍历的下一个数
• 所以就是求BST的中序遍历
• 这里使用一个栈来实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class BSTIterator {
    Stack<TreeNode> stk;
    public BSTIterator(TreeNode root) {
        stk = new Stack<>();
        while(root != null){
            stk.push(root);
            root = root.left;
        }
    }

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

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