HDU 5171 GTY's birthday gift（矩阵快速幂 ）

#include <cstdio>    
#include <cstring>
#include <algorithm>
using namespace std;
#define MAX_SIZE 3
#define MOD 10000007
#define clr( a, b ) memset( a, b, sizeof(a) )
typedef long long LL;

struct Mat
{
    int n;
    LL mat[MAX_SIZE][MAX_SIZE];
    Mat( int _n )
    {
        n = _n;
        clr( mat, 0 );
    }
    void init()
    {
        for( int i = 0; i < n; ++i )
            for( int j = 0; j < n; ++j )
                mat[i][j] = ( i == j );
    }
    Mat operator * ( const Mat &b ) const
    {
        Mat c( b.n );
        for( int k = 0; k < n; ++k )
            for( int i = 0; i < n; ++i )    if( mat[i][k] )
                for( int j = 0; j < n; ++j )
                    c.mat[i][j] = ( c.mat[i][j] + mat[i][k] * b.mat[k][j] ) % MOD;
        return c;
    }
};

Mat fast_mod( Mat a, int b )
{
    Mat res( a.n );
    res.init();
    while( b )
    {
        if( b & 1 )    res = res * a;
        a = a * a;
        b >>= 1; 
    }
    return res;
}

void scan( int &x )
{
    char c;
    while( c = getchar(), c < ‘0‘ || c > ‘9‘ );
    x = c - ‘0‘;
    while( c = getchar(), c >= ‘0‘ && c <= ‘9‘ ) x = x *10 + c - ‘0‘;
}

int n, k, x[ 100001 ], id;
int first, second;
LL sum, c[ 3 ], res;

int main()
{
    Mat M( 3 );
    M.mat[0][0] = M.mat[0][2] = M.mat[1][2] = M.mat[2][1] = M.mat[2][2] = 1;
    while( ~scanf( "%d %d", &n, &k ) )
    {
        sum = 0;
        first = second = -1;
        for( int i = 0; i < n; ++i )
        {
            scan( x[i] );
            sum += x[i];
            if( sum > MOD ) sum -= MOD; 
            if( x[i] > first )
            {
                first = x[i];
                id = i;
            }
        }
        for( int i = 0; i < n; ++i )
        {
            if( i == id ) continue;
            if( x[i] > second ) second = x[i];
        }
        c[ 0 ] = sum - first;
        c[ 1 ] = second;
        c[ 2 ] = first;
    //    printf( "c[0] = %lld c[1] =  %lld c[2] = %lld\n", c[0],c[1],c[2] );
        Mat b = fast_mod( M, k + 1 );
        res = ( c[ 0 ] *  b.mat[0][0] + c[ 1 ] * b.mat[0][1] + c[ 2 ] * b.mat[0][2] ) % MOD ;
        printf( "%I64d\n", res );
        
    }
    return 0;
}