思路

用栈模拟中序遍历的代码进行拆分；

每次加子树就是把左子树最左边的一条链加进来；

遍历完左子树，要把右子树加进来。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
from collections import deque
class BSTIterator:

    def __init__(self, root: TreeNode):
        self.stk = deque()

        # 把根节点最左边的一条链加进来
        while root:
            self.stk.append(root)
            root = root.left



    def next(self) -> int:
        """
        @return the next smallest number
        """
        p = self.stk.pop() # 后进先出，把栈顶元素弹出
        res = p.val 
        p = p.right # 把右子树的最左边的一条链加进来 
        while p:
            self.stk.append(p)
            p = p.left 
        return res 

    def hasNext(self) -> bool:
        """
        @return whether we have a next smallest number
        """
        return len(self.stk) > 0



# Your BSTIterator object will be instantiated and called as such:
# obj = BSTIterator(root)
# param_1 = obj.next()
# param_2 = obj.hasNext()

# 用栈模拟中序遍历的代码进行拆分
# 每次加子树就是把左子树最左边的一条链加进来
# 遍历完左子树，要把右子树加进来