C++,Python排列解Leetcode難題1359Count All Valid Pickup and Delivery Options

這個Leetcode難題是很好的中學數學問題，排列問題。

 
[codes on Leetcode] https://leetcode.com/problems/count-all-valid-pickup-and-delivery-options/solutions/4024293/c-python-math-recursion-1-loop/

class Solution:
    def countOrders(self, n: int) -> int:
        mod=10**9+7
        num=1
        for i in range(1, n):
            num=num*(i+1)*(2*i+1)%mod
        return num

有一行解的。