# Student Attendance Record II

This page explains Java solution to problem `Student Attendance Record II` using `Dynamic Programming`.

## Problem Statement

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:

• 'A' : Absent.
• 'L' : Late.
• '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
Explanation:
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.

Example 2:

Input: n = 6
Output: 200

## Solution

If you have any suggestions in below code, please create a pull request by clicking here.
``````
package com.vc.hard;

class StudentAttendanceRecordIi {
public int checkRecord(int n) {
int MOD = (int)(1e9 + 7);

if(n == 1) return 3;
else if(n == 2) return 8;

/**
P = Number of sequences ending in 'P'
A = Number of sequences ending in 'A'
L = Number of sequences ending in 'L'

P_NO_A = Number of sequences ending in 'P' and doesn't have 'A'
L_NO_A = Number of sequences ending in 'L' and doesn't have 'A'
*/
long[] P = new long[n + 1];
long[] A = new long[n + 1];
long[] L = new long[n + 1];
long[] P_NO_A = new long[n + 1];
long[] L_NO_A = new long[n + 1];

/**
When i = 1

Number of sequences ending in 'P'
P

Number of sequences ending in 'A'
A

Number of sequences ending in 'L'
L

Number of sequences ending in 'P' and doesn't have 'A'
P

Number of sequences ending in 'L' and doesn't have 'A'
L
*/
P = 1;
A = 1;
L = 1;
P_NO_A = 1;
L_NO_A = 1;

/**
When i = 2

Number of sequences ending in 'P'
PP
LP
AP

Number of sequences ending in 'A'
PA
LA
Not AA because more than one 'A' is not regarded as rewardable

Number of sequences ending in 'L'
PL
AL
LL

Number of sequences ending in 'P' and doesn't have 'A'
PP
LP

Number of sequences ending in 'L' and doesn't have 'A'
PL
LL
*/
P = 3;
A = 2;
L = 3;
P_NO_A = 2;
L_NO_A = 2;

/**
When i = 3

Number of sequences ending in 'P'
PPP
APP
LPP
PAP
Not AAP because more than one 'A' is not regarded as rewardable
LAP
PLP
ALP
LLP

Number of sequences ending in 'A'
PPA
Not APA because more than one 'A' is not regarded as rewardable
LPA
Not PAA because more than one 'A' is not regarded as rewardable
Not AAA because more than one 'A' is not regarded as rewardable
Not LAA because more than one 'A' is not regarded as rewardable
PLA
Not ALA because more than one 'A' is not regarded as rewardable
LLA

Number of sequences ending in 'L'
PPL
APL
LPL
PAL
Not AAL because more than one 'A' is not regarded as rewardable
LAL
PLL
ALL
Not LLL because more than two continuous 'L' is not regarded as rewardable

Number of sequences ending in 'P' and doesn't have 'A'
PPP
Not APP because it has A
LPP
Not PAP because it has A
Not AAP because it has A
Not LAP because it has A
PLP
Not ALP because it has A
LLP

Number of sequences ending in 'L' and doesn't have 'A'
PPL
Not APL because it has A
LPL
Not PAL because it has A
Not AAL because it has A
Not LAL because it has A
PLL
Not ALL because it has A
Not LLL because it has more than two continuous 'L'
*/
P = 8;
A = 4;
L = 7;
P_NO_A = 4;
L_NO_A = 3;

/**
Rule for P:                                                               You can append P to any Seq
P(n) = P(n - 1) + L(n - 1) + A(n - 1)

Rule for L: There can't be more than two continuous L
L(n) =  P(n - 1)                                                          You can append L to Sequences ending in P
+ A(n - 1)                                                          You can append L to Sequences ending in A
+ if(n - 2 == 'A' || n - 2 == 'P') P(n - 2) + A(n - 2)              You can append L to Sequence, if Prev To Prev Character is NOT L

Rule for A: There can't be more than one A
A(n) =   P_NO_A(n - 1)                                                    You can append A to Sequences ending in P and has no 'A'
+ L_NO_A(n - 1)                                                    You can append A to Sequences ending in L and has no 'A'

P_NO_A(n) = A(n)                                                          You can append P to any Seq
L_NO_A(n) = A(n - 1) + A(n - 2)
*/
for(int i = 4; i <= n; i++) {
P[i] = (P[i - 1] + L[i - 1] + A[i - 1]) % MOD;
L[i] = (P[i - 1] + A[i - 1] + P[i - 2] + A[i - 2]) % MOD;
A[i] = (P_NO_A[i - 1] + L_NO_A[i - 1]) % MOD;

P_NO_A[i] = A[i];
L_NO_A[i] = (A[i - 1] + A[i - 2]) % MOD;
}

return (int)((P[n] + L[n] + A[n]) % MOD);
}
}
``````

## Time Complexity

O(N) Where
N is given positive integer

## Space Complexity

O(N) Where
N is given positive integer