README.md

1359. 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

Solutions (Rust)

1. Solution

impl Solution {
    pub fn count_orders(n: i32) -> i32 {
        let mut ret = 1_i64;

        for i in 1..n as i64 {
            ret = (ret * (2 * i + 1) * (i + 1)) % 1_000_000_007;
        }

        ret as i32
    }
}