95题
输出所有可能的搜索二叉树
采用递归，不断地进行root和左右子节点的递归

class Solution(object):
    def generateTrees(self, n):
        if n == 0: return []
            
        def helper(start,end):
            res = []
            if start > end:
                res.append(None)
            for val in range(start, end + 1):
                left = helper(start, val - 1)
                right = helper(val + 1,end)
                for i in left:
                    for j in right:
                        root = TreeNode(val)
                        root.left = i
                        root.right = j
                        res.append(root)
            return res
                    
        return helper(1, n)

96题采用动态规划

class Solution(object):
    def numTrees(self, n):
        dp = [0] * (n + 1)
        dp[0] = 1
        dp[1] = 1
        
        for i in range(2, n + 1):
            for j in range(1, i + 1):
                dp[i] += dp[j - 1] * dp[i - j]
        return dp[-1]