Unique Binary Search Trees.py

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