student-attendance-record-ii


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Points: 5
Time limit: 30.0s
Memory limit: 250M

Problem type
Allowed languages
Python

An attendance record for a student can be represented as a string where each character signifies whether the student was absent, late, or present on that day. The record only contains the following three characters:

'A': Absent.
'L': Late.
'P': Present.

Any student is eligible for an attendance award if they meet both of the following criteria:

The student was absent ('A') for strictly fewer than 2 days total.
The student was never late ('L') for 3 or more consecutive days.

Given an integer n, return the number of possible attendance records of length n that make a student eligible for an attendance award. The answer may be very large, so return it modulo 10⁹ + 7.

  Example 1:

Input: n = 2 Output: 8 Explanation: There are 8 records with length 2 that are eligible for an award: "PP", "AP", "PA", "LP", "PL", "AL", "LA", "LL" Only "AA" is not eligible because there are 2 absences (there need to be fewer than 2).

Example 2:

Input: n = 1 Output: 3

Example 3:

Input: n = 10101 Output: 183236316

  Constraints:

1 <= n <= 10⁵

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