矩阵快速幂

https://vjudge.net/contest/360957#problem/J

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
struct Matrix{
    int m[2][2];
};
Matrix mul(Matrix a,Matrix b){
    Matrix ans;
    for(int i = 0; i < 2; i++){
        for(int j = 0; j < 2; j++){     
            ans.m[i][j] = 0;
            for(int k = 0; k < 2; k++){
                ans.m[i][j] = (ans.m[i][j] + a.m[i][k] * b.m[k][j] % 7) % 7;
            }

        }
    }
    return ans;
}
int main(){
    int A,B,n;
    while(scanf("%d%d%d",&A,&B,&n) != EOF){
        if(A == 0 && B == 0 && n == 0) break;
        if(n <= 2){
            printf("1
");
            continue;
        }
        Matrix a,ans;
        a.m[0][0] = A;
        a.m[0][1] = B;
        a.m[1][0] = 1;
        a.m[1][1] = 0;
        ans.m[0][0] = ans.m[1][1] = 1;
        ans.m[0][1] = ans.m[1][0] = 0;
        n -= 2;
        while(n){
            if(n & 1)
                ans = mul(ans,a);
            n >>= 1;
            a = mul(a,a);
        }
        printf("%d
",(ans.m[0][0] + ans.m[0][1]) % 7);
    }
    return 0;
}