Today I have done leetcode's September LeetCoding Challenge with cpp.

September LeetCoding Challenge 10

Description

Count All Valid Pickup and Delivery Options

Given n orders, each order consist in pickup and delivery services. 

Count all valid pickup/delivery possible sequences such that delivery(i) is always after of pickup(i). 

Since the answer may be too large, return it modulo 10^9 + 7.

Example 1:

Input: n = 1
Output: 1
Explanation: Unique order (P1, D1), Delivery 1 always is after of Pickup 1.

Example 2:

Input: n = 2
Output: 6
Explanation: All possible orders: 
(P1,P2,D1,D2), (P1,P2,D2,D1), (P1,D1,P2,D2), (P2,P1,D1,D2), (P2,P1,D2,D1) and (P2,D2,P1,D1).
This is an invalid order (P1,D2,P2,D1) because Pickup 2 is after of Delivery 2.

Example 3:

Input: n = 3
Output: 90

Constraints:

• 1 <= n <= 500

Solution

const int MOD = 1e9 + 7;
constexpr auto table = []{
  array<int, 501> answer{0, 1};
  for(int i = 2; i <= 500; ++i) {
    answer[i] = 1LL * answer[i - 1] * i * (i * 2 - 1) % MOD;
  }
  return answer;
}();
class Solution {
public:
  int countOrders(int n) {
    return table[n];
  }
};

// Accepted
// 36/36 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 31.45 % of cpp submissions (6 MB)