leetcode_flatten binary tree to linked list

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class :
    def flatten(self, root: TreeNode) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
        if not root:
            return
        
        if root.left:
            self.flatten(root.left)
        if root.right:
            self.flatten(root.right)
        
        # move the left tree to the right, set left to None 
        p_temp = root.right
        root.right = root.left
        root.left = None
        
        # find the leaf and link the right tree
        p_tail = root
        while p_tail.right:
            p_tail = p_tail.right
        p_tail.right = p_temp
        
        return