text: class Solution:
      # @paramn n: An integer
      # @return: An integer
      def numTrees(self, n):
          dp = [1,1]
          # write your code here
          for i in range(2, n + 1):
              sum = 0
              for j in range(1, i + 1):
                  left = j - 1
                  right = i - j
                  sum += dp[left] * dp[right]
              dp.append(sum)
          return dp[n]
  