# LeetCode 1359. Count All Valid Pickup and Delivery Options

## Question

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 |

## Solution

We assumed it already have `n - 1`

pair, and we have to place the final pair. The number of we can place is `2 * (n - 1) + 1 = 2*n - 1`

, so for the last, it should be `2 * n`

, which add one more section we can use due to the operation.

## Code

1 | // time:O(n), space:O(1) |