230. 二叉搜索树中第 K 小的元素

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        cnt =0 
        s =[]
        cur =root 
        while cur or  s is not None:
            while cur :
                s.append(cur)
                cur =cur.left 
            cur =s.pop()  #1.left
            cnt +=1
            if(cnt ==k):
                return cur.val 
            cur =cur.right 
        return -1