Codeforces 450B Jzzhu and Sequences(矩阵快速幂)

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
typedef long long ll;
const ll mod = 1e9+7;

ll s[2][2] = {1, 0, 0, 1};

void mat_mod (ll a[2][2], ll b[2][2]) {
ll c[2][2];
memset(c, 0, sizeof(c));
for (int k = 0; k < 2; k++) {
for (int i = 0; i < 2; i++)
for (int j = 0; j < 2; j++)
c[i][j] = ((c[i][j] + a[i][k] * b[k][j]) % mod + mod) % mod;
}
memcpy(a, c, sizeof(c));
}

void mat_pow(int n) {
ll x[2][2] = {0, 1, -1, 1};
while (n) {
if (n&1)
mat_mod(s, x);
mat_mod(x, x);
n >>= 1;
}
}

int main () {
int x, y, n;
scanf("%d%d%d", &x, &y, &n);
if (n == 1)
printf("%lld\n", (x % mod + mod) % mod);
else {
mat_pow(n-2);
printf("%lld\n", ((x * s[1][0] % mod + y * s[1][1] % mod) % mod + mod) % mod);
}
return 0;
}