538. Convert BST to Greater Tree

class Solution(object):
    def convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        a=[]
        def dfs(root):
            if not root: return
            dfs(root.left)
            a.append(root.val)
            dfs(root.right)
        dfs(root)
        
        for i in range(len(a)-2,-1,-1):
            a[i]+=a[i+1]
            
        i = [0]
        def dfs2(root):
            if not root: return
            dfs2(root.left)
            root.val = a[i[0]]
            i[0]+=1
            dfs2(root.right)
        dfs2(root)
        
        return root