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# 1227. Airplane Seat Assignment Probability

<https://leetcode.com/problems/airplane-seat-assignment-probability>

## Description

`n` passengers board an airplane with exactly `n` seats. The first passenger has lost the ticket and picks a seat randomly. But after that, the rest of passengers will:

* Take their own seat if it is still available,
* Pick other seats randomly when they find their seat occupied

What is the probability that the n-th person can get his own seat?

**Example 1:**

```

**Input:** n = 1
**Output:** 1.00000
**Explanation:** The first person can only get the first seat.
```

**Example 2:**

```

**Input:** n = 2
**Output:** 0.50000
**Explanation:** The second person has a probability of 0.5 to get the second seat (when first person gets the first seat).
```

**Constraints:**

* `1 <= n <= 10^5`

## ac

```java
```