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1227. 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 the passengers will:
 Take their own seat if it is still available, and
 Pick other seats randomly when they find their seat occupied
Return the probability that the n^{th}
person gets 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}
Solutions
Solution 1: Mathematics
Let $f(n)$ represent the probability that the $n$th passenger will sit in their own seat when there are $n$ passengers boarding. Consider from the simplest case:

When $n=1$, there is only 1 passenger and 1 seat, so the first passenger can only sit in the first seat, $f(1)=1$;

When $n=2$, there are 2 seats, each seat has a probability of 0.5 to be chosen by the first passenger. After the first passenger chooses a seat, the second passenger can only choose the remaining seat, so the second passenger has a probability of 0.5 to sit in their own seat, $f(2)=0.5$.
When $n>2$, how to calculate the value of $f(n)$? Consider the seat chosen by the first passenger, there are three cases.

The first passenger has a probability of $\frac{1}{n}$ to choose the first seat, then all passengers can sit in their own seats, so the probability of the $n$th passenger sitting in their own seat is 1.0.

The first passenger has a probability of $\frac{1}{n}$ to choose the $n$th seat, then the second to the $(n1)$th passengers can sit in their own seats, the $n$th passenger can only sit in the first seat, so the probability of the $n$th passenger sitting in their own seat is 0.0.

The first passenger has a probability of $\frac{n2}{n}$ to choose the remaining seats, each seat has a probability of $\frac{1}{n}$ to be chosen. Suppose the first passenger chooses the $i$th seat, where $2 \le i \le n1$, then the second to the $(i1)$th passengers can sit in their own seats, the seats of the $i$th to the $n$th passengers are uncertain, the $i$th passenger will randomly choose from the remaining $n(i1)=ni+1$ seats (including the first seat and the $(i+1)$th to the $n$th seats). Since the number of remaining passengers and seats is $ni+1$, and 1 passenger will randomly choose a seat, the problem size is reduced from $n$ to $ni+1$.
Combining the above three cases, we can get the recursive formula of $f(n)$:
\[\begin{aligned} f(n) &= \frac{1}{n} \times 1.0 + \frac{1}{n} \times 0.0 + \frac{1}{n} \times \sum_{i=2}^{n1} f(ni+1) \\ &= \frac{1}{n}(1.0+\sum_{i=2}^{n1} f(ni+1)) \end{aligned}\]In the above recursive formula, there are $n2$ values of $i$, since the number of values of $i$ must be a nonnegative integer, so the above recursive formula only holds when $n2 \ge 0$ i.e., $n \ge 2$.
If you directly use the above recursive formula to calculate the value of $f(n)$, the time complexity is $O(n^2)$, which cannot pass all test cases, so it needs to be optimized.
Replace $n$ with $n1$ in the above recursive formula, you can get the recursive formula:
\[f(n1) = \frac{1}{n1}(1.0+\sum_{i=2}^{n2} f(ni))\]In the above recursive formula, there are $n3$ values of $i$, and the above recursive formula only holds when $n3 \ge 0$ i.e., $n \ge 3$.
When $n \ge 3$, the above two recursive formulas can be written as follows:
\[\begin{aligned} n \times f(n) &= 1.0+\sum_{i=2}^{n1} f(ni+1) \\ (n1) \times f(n1) &= 1.0+\sum_{i=2}^{n2} f(ni) \end{aligned}\]Subtract the above two formulas:
\[\begin{aligned} &~~~~~ n \times f(n)  (n1) \times f(n1) \\ &= (1.0+\sum_{i=2}^{n1} f(ni+1))  (1.0+\sum_{i=2}^{n2} f(ni)) \\ &= \sum_{i=2}^{n1} f(ni+1)  \sum_{i=2}^{n2} f(ni) \\ &= f(2)+f(3)+...+f(n1)  (f(2)+f(3)+...+f(n2)) \\ &= f(n1) \end{aligned}\]After simplification, we get the simplified recursive formula:
\[\begin{aligned} n \times f(n) &= n \times f(n1) \\ f(n) &= f(n1) \end{aligned}\]Since we know that $f(1)=1$ and $f(2)=0.5$, therefore when $n \ge 3$, according to $f(n) = f(n1)$, we know that for any positive integer $n$, $f(n)=0.5$. And since $f(2)=0.5$, therefore when $n \ge 2$, for any positive integer $n$, $f(n)=0.5$.
From this, we can get the result of $f(n)$:
\[f(n) = \begin{cases} 1.0, & n = 1 \\ 0.5, & n \ge 2 \end{cases}\]
class Solution { public double nthPersonGetsNthSeat(int n) { return n == 1 ? 1 : .5; } }

class Solution { public: double nthPersonGetsNthSeat(int n) { return n == 1 ? 1 : .5; } };

class Solution: def nthPersonGetsNthSeat(self, n: int) > float: return 1 if n == 1 else 0.5

func nthPersonGetsNthSeat(n int) float64 { if n == 1 { return 1 } return .5 }