Leetcode Problem 552. Student Attendance Record II

552. Student Attendance Record II

Leetcode Solutions

Approach # Dynamic Programming with Constant Space

  1. Initialize six variables to represent the states: P, L, LL, A, AL, ALL, where P is the count of strings ending with 'P', L with 'L', LL with two 'L's, A with 'A', AL with 'A' and 'L', and ALL with 'A' and two 'L's.
  2. Set initial values: P = 1, L = 1, LL = 0, A = 1, AL = 0, ALL = 0.
  3. Iterate from 2 to n (inclusive), updating the counts for each state based on the previous values.
  4. At each iteration, calculate the new counts for P, L, LL, A, AL, and ALL using the state transition rules.
  5. After the loop, sum up P, L, LL, A, AL, and ALL to get the total number of valid attendance records.
  6. Return the total count modulo 10^9 + 7.
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Approach # Using Recursive Formulae

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