[leetcode tree]95. Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

   1         3     3      2      1
           /     /      /       
     3     2     1      1   3      2
    /     /                        
   2     1         2                 3



输入n,求出所有1-n组成的所有可能的树

class Solution(object):
    def generateTrees(self, n):
        def node(v,l,r):
            n = TreeNode(v)
            n.left = l
            n.right = r
            return n
        def tree(first,last):
            return [node(v,l,r) for v in range(first,last+1)
                                for l in tree(first,v-1)
                                for r in tree(v+1,last)] or [None]
        return tree(1,n) if n else []