95. Unique Binary Search Trees II

这一题看了几个做法都是递归哎

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

class Solution(object):
    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        if n == 0:
            return []
        return self.generate(1, n)
    def generate(self,start,end):
        if start > end:
            return [None]
        if start == end:
            return [TreeNode(start)]
        res = []
        for i in range(start, end+1):
            leftnode = self.generate(start, i-1)
            rightnode = self.generate(i+1, end)
            for l in leftnode:
                for r in rightnode:
                    root = TreeNode(i)
                    root.left = l
                    root.right = r
                    res.append(root)
        return res