def numTrees_math(self, n):
        return math.factorial(2*n)/(math.factorial(n)*math.factorial(n))-math.factorial(2*n)/(
            math.factorial(n+1)*math.factorial(n-1))
number of unique binary search tree
dataset/ETHPy150Open algorhythms/LeetCode/094 Unique Binary Search Trees.py/Solution.numTrees_math