Leetcode: Student Attendance Record II

Student Attendance Record II

Similar Problems:

Given a positive integer n, return the number of all possible attendance records with length n, which will be regarded as rewardable. The answer may be very large, return it after mod 109 + 7.

A student attendance record is a string that only contains the following three characters:

  1. ‘A’ : Absent.
  2. ‘L’ : Late.
  3. ‘P’ : Present.

A record is regarded as rewardable if it doesn’t contain more than one ‘A’ (absent) or more than two continuous ‘L’ (late).

Example 1:

Input: n = 2
Output: 8 
There are 8 records with length 2 will be regarded as rewardable:
"PP" , "AP", "PA", "LP", "PL", "AL", "LA", "LL"
Only "AA" won't be regarded as rewardable owing to more than one absent times. 

Note: The value of n won’t exceed 100,000.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## Blog link: https://code.dennyzhang.com/student-attendance-record-ii
## Basic Ideas: Dynamic Programming
##      a0l0, a0l1, a0l2
##      a1l0, a1l1, a1l2
## Complexity: Time O(n), Space O(1)
class Solution:
    def checkRecord(self, n):
        :type n: int
        :rtype: int
        if n == 1: return 3
        l = [1, 1, 0, 1, 0, 0]
        for i in range(2, n+1):
            l2 = [None]*6
            # a0l0: first value is P
            l2[0] = l[0]
            # a0l1: first value is P or L
            l2[1] = l[1] + l[0]
            # a0l2: first value is P or L
            l2[2] = l[2] + l[1]
            # a1l0: first value is A or P
            l2[3] = l[0] + l[3]
            # a1l1: first value is A or P or L
            l2[4] = l[1] + l[4] + l[3]
            # a1l2: first value is A or P or L
            l2[5] = l[2] + l[5] + l[4]
            l = l2
        return sum(l)

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