Leetcode: Unique Binary Search Trees II @Python

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

class Solution:
    #@return a list of tree node with value range from start to end
    def TreeGen(self, start,end):
        if start>end:
            return [None]
        rst=[]
        for rVal in range(start, end+1):
            leftlist=self.TreeGen(start,rVal-1)
            rightlist=self.TreeGen(rVal+1,end)
            for i in leftlist:
                for j in rightlist:
                    root=TreeNode(rVal)
                    root.left=i
                    root.right=j
                    rst.append(root)
        return rst
            
        
    # @return a list of tree node    
    def generateTrees(self, n):
        return self.TreeGen(1,n)