CodeForces 450B Jzzhu and Sequences

矩阵快速幂。

首先得到公式

然后构造矩阵，用矩阵加速

取模函数需要自己写一下，是数论中的取模。

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<algorithm>
using namespace std;
 
long long MOD = 1e9 + ;
long long x, y;
int n;
 
long long mod(long long a, long long b)
{
    if (a >= ) return a%b;
    if (abs(a) % b == ) return ;
    return (a + b*(abs(a) / b + ));
}
 
struct Matrix
{
    long long A[][];
    int R, C;
    Matrix operator*(Matrix b);
};
 
Matrix X, Y, Z;
 
Matrix Matrix::operator*(Matrix b)
{
    Matrix c;
    memset(c.A, , sizeof(c.A));
    int i, j, k;
    for (i = ; i <= R; i++)
        for (j = ; j <= C; j++)
            for (k = ; k <= C; k++)
                c.A[i][j] = mod((c.A[i][j] + mod(A[i][k] * b.A[k][j], MOD)), MOD);
    c.R=R; c.C=b.C;
    return c;
}
 
void read()
{
    scanf("%lld%lld%d", &x, &y, &n);
}
 
void init()
{
    n = n - ;
    Z.A[][] = x, Z.A[][] = y; Z.R = ; Z.C = ;
    Y.A[][] = , Y.A[][] = , Y.A[][] = , Y.A[][] = ; Y.R = ; Y.C = ;
    X.A[][] = , X.A[][] = -, X.A[][] = , X.A[][] = ; X.R = ; X.C = ;
}
 
void work()
{
    while (n)
    {
        if (n %  == ) Y = Y*X;
        n = n >> ;
        X = X*X;
    }
    Z = Z*Y;
 
    printf("%lld\n", mod(Z.A[][], MOD));
}
 
int main()
{
    read();
    init();
    work();
    return ;
}