96. Unique Binary Search Trees

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST's.

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

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