Leetcode 96 Unique Binary Search Trees

class Solution:
    # @param {integer} n
    # @return {integer}
    def numTrees(self, n):
        dp = [0]*(n+1); dp[0] = 1
        for nodeNum in range(1,n+1):
            for k in range(nodeNum):
                dp[nodeNum] += dp[k] * dp[nodeNum - 1 - k]
        return dp[n]