competitive_programming_codebook/MatExpo at main · soura07/competitive_programming_codebook · GitHub

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void multiply(int a[3][3], int b[3][3])
{
    int mul[3][3];
    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
            mul[i][j] = 0;
            for (int k = 0; k < 3; k++)
                mul[i][j] += a[i][k]*b[k][j];
        }
    }
    for (int i=0; i<3; i++)
        for (int j=0; j<3; j++)
            a[i][j] = mul[i][j];
}
int power(int F[3][3], int n)
{
    int M[3][3] = {{1,1,1}, {1,0,0}, {0,1,0}};
    if (n==1)
        return F[0][0] + F[0][1];
    power(F, n/2);
    multiply(F, F);
    if (n%2 != 0)
        multiply(F, M);
    return F[0][0] + F[0][1] ;
}
int findNthTerm(int n)
{
    int F[3][3] = {{1,1,1}, {1,0,0}, {0,1,0}} ;
    return power(F, n-2);
}