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class Solution(object):
    def numTrees(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n == 0:
            return 0
        recd = [0]*(n+1)
        # There are recd[i] ways to organize a tree with i nodes.
        recd[0] = 1
        recd[1] = 1
        for i in range(2,n+1):
            for j in range(1,i+1):
                # j as the root node of the BST
                # The left tree has `j-1` nodes and the right tree has `i-j` nodes.
                recd[i] += recd[j-1] * recd[i-j]
        return recd[n]