Student Attendance Record II — LeetCode 552 Python Solution

The problem

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.

Example

Input

n = 2

Output

8

Explanation

There are 8 records with length 2 that are eligible for an award:

Python solution

class Solution:
    def checkRecord(self, n: int) -> int:
        @cache
        def dfs(i, j, k):
            if i >= n:
                return 1
            ans = 0
            if j == 0:
                ans += dfs(i + 1, j + 1, 0)
            if k < 2:
                ans += dfs(i + 1, j, k + 1)
            ans += dfs(i + 1, j, 0)
            return ans % mod

        mod = 10**9 + 7
        ans = dfs(0, 0, 0)
        dfs.cache_clear()
        return ans

Complexity

Pattern: Dynamic Programming

Define a state, write the transition, and stop recomputing the same subproblem. LeetCode 552. Student Attendance Record II is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Dynamic Programming.

The dynamic programming guide has the Python template for the pattern and the 481 LeetCode problems that use it.

Related problems