class Solution:
    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        if not root: return root
        stack = []
        result = []
        cur = root
        pre = 0
        while cur or stack:
            if cur:
                stack.append(cur)
                cur = cur.right
            else: 
                cur = stack.pop()
                cur.val+= pre
                pre = cur.val
                cur =cur.left
        return root