2954. Count the Number of Infection Sequences
Description
You are given an integer n
and a 0indexed integer array sick
which is sorted in increasing order.
There are n
children standing in a queue with positions 0
to n  1
assigned to them. The array sick
contains the positions of the children who are infected with an infectious disease. An infected child at position i
can spread the disease to either of its immediate neighboring children at positions i  1
and i + 1
if they exist and are currently not infected. At most one child who was previously not infected can get infected with the disease in one second.
It can be shown that after a finite number of seconds, all the children in the queue will get infected with the disease. An infection sequence is the sequential order of positions in which all of the noninfected children get infected with the disease. Return the total number of possible infection sequences.
Since the answer may be large, return it modulo 10^{9} + 7
.
Note that an infection sequence does not contain positions of children who were already infected with the disease in the beginning.
Example 1:
Input: n = 5, sick = [0,4] Output: 4 Explanation: Children at positions 1, 2, and 3 are not infected in the beginning. There are 4 possible infection sequences:  The children at positions 1 and 3 can get infected since their positions are adjacent to the infected children 0 and 4. The child at position 1 gets infected first. Now, the child at position 2 is adjacent to the child at position 1 who is infected and the child at position 3 is adjacent to the child at position 4 who is infected, hence either of them can get infected. The child at position 2 gets infected. Finally, the child at position 3 gets infected because it is adjacent to children at positions 2 and 4 who are infected. The infection sequence is [1,2,3].  The children at positions 1 and 3 can get infected because their positions are adjacent to the infected children 0 and 4. The child at position 1 gets infected first. Now, the child at position 2 is adjacent to the child at position 1 who is infected and the child at position 3 is adjacent to the child at position 4 who is infected, hence either of them can get infected. The child at position 3 gets infected. Finally, the child at position 2 gets infected because it is adjacent to children at positions 1 and 3 who are infected. The infection sequence is [1,3,2].  The infection sequence is [3,1,2]. The order of infection of disease in the children can be seen as: [0,1,2,3,4] => [0,1,2,3,4] => [0,1,2,3,4] => [0,1,2,3,4].  The infection sequence is [3,2,1]. The order of infection of disease in the children can be seen as: [0,1,2,3,4] => [0,1,2,3,4] => [0,1,2,3,4] => [0,1,2,3,4].
Example 2:
Input: n = 4, sick = [1] Output: 3 Explanation: Children at positions 0, 2, and 3 are not infected in the beginning. There are 3 possible infection sequences:  The infection sequence is [0,2,3]. The order of infection of disease in the children can be seen as: [0,1,2,3] => [0,1,2,3] => [0,1,2,3] => [0,1,2,3].  The infection sequence is [2,0,3]. The order of infection of disease in the children can be seen as: [0,1,2,3] => [0,1,2,3] => [0,1,2,3] => [0,1,2,3].  The infection sequence is [2,3,0]. The order of infection of disease in the children can be seen as: [0,1,2,3] => [0,1,2,3] => [0,1,2,3] => [0,1,2,3].
Constraints:
2 <= n <= 10^{5}
1 <= sick.length <= n  1
0 <= sick[i] <= n  1
sick
is sorted in increasing order.
Solutions
Solution 1: Combinatorial Mathematics + Multiplicative Inverse + Fast Power
According to the problem description, the children who have a cold have divided the children who have not yet caught a cold into several continuous segments. We can use an array $nums$ to record the number of children who are not cold in each segment, and there are a total of $s = \sum_{i=0}^{k} nums[k]$ children who are not cold. We can find that the number of cold sequences is the number of permutations of $s$ different elements, that is, $s!$.
Assuming that there is only one transmission scheme for each segment of children who are not cold, there are $\frac{s!}{\prod_{i=0}^{k} nums[k]!}$ cold sequences in total.
Next, we consider the transmission scheme of each segment of children who are not cold. Suppose there are $x$ children in a segment who are not cold, then they have $2^{x1}$ transmission schemes, because each time you can choose one end from the left and right ends of a segment to transmit, that is: two choices, there are a total of $x1$ transmissions. However, if it is the first segment or the last segment, there is only one choice.
In summary, the total number of cold sequences is:
$$ \frac{s!}{\prod_{i=0}^{k} nums[k]!} \prod_{i=1}^{k1} 2^{nums[i]1} $$
Finally, we need to consider that the answer may be very large and need to be modulo $10^9 + 7$. Therefore, we need to preprocess the factorial and multiplicative inverse.
The time complexity is $O(m)$, where $m$ is the length of the array $sick$. Ignoring the space consumption of the preprocessing array, the space complexity is $O(m)$.









