java 552.学生出勤记录II（1st）.java

Posted 2021-05-08

public class Solution {
    static final int M = 1000000007;

    public int checkRecord(int n) {
        long[] PorL = new long[n + 1]; // ending with P or L, no A
        long[] P = new long[n + 1]; // ending with P, no A
        PorL[0] = P[0] = 1; PorL[1] = 2; P[1] = 1;

        for (int i = 2; i <= n; i++) {
            P[i] = PorL[i - 1];
            PorL[i] = (P[i] + P[i - 1] + P[i - 2]) % M;
        }

        long res = PorL[n];
        for (int i = 0; i < n; i++) { // inserting A into (n-1)-length strings
            long s = (PorL[i] * PorL[n - i - 1]) % M;
            res = (res + s) % M;
        }

        return (int) res;
    }
}

public class Solution {
final int MOD = 1000000007;
final int M = 6;

int[][] mul(int[][] A, int[][] B) {
    int[][] C = new int[M][M];
    for (int i = 0; i < M; i++)
        for (int j = 0; j < M; j++)
            for (int k = 0; k < M; k++)
                C[i][j] = (int) ((C[i][j] + (long) A[i][k] * B[k][j]) % MOD);
    return C;
}


int[][] pow(int[][] A, int n) {
    int[][] res = new int[M][M];
    for (int i = 0; i < M; i++)
        res[i][i] = 1;
    while (n > 0) {
        if (n % 2 == 1)
            res = mul(res, A);
        A = mul(A, A);
        n /= 2;
    }
    return res;
}

public int checkRecord(int n) {
    int[][] A = {
            {0, 0, 1, 0, 0, 0},
            {1, 0, 1, 0, 0, 0},
            {0, 1, 1, 0, 0, 0},
            {0, 0, 1, 0, 0, 1},
            {0, 0, 1, 1, 0, 1},
            {0, 0, 1, 0, 1, 1},
    };
    return pow(A, n + 1)[5][2];
}
}