# 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
    def dfs(self, start, end):
        if start > end: return [None]
        res = []
        for rootval in range(start, end+1):　　　　　　　　#rootval为根节点的值，从start遍历到end
            LeftTree = self.dfs(start, rootval-1)
            RightTree = self.dfs(rootval+1, end)
            for i in LeftTree:　　　　　　　　　　　　　　　　#i遍历符合条件的左子树
                for j in RightTree:　　　　　　　　　　　　  #j遍历符合条件的右子树
                    root = TreeNode(rootval)
                    root.left = i
                    root.right = j
                    res.append(root)
        return res
    def generateTrees(self, n):
        return self.dfs(1, n)