输入A B n ，f(1) = 1, f(2) = 1, f(n) = (A * f(n - 1) + B * f(n - 2)) mod 7 难点：1.题目n会达到10亿，如果单纯用递归，空间不够用，所以得发现规律 #include
    
     using namespace std;int f(int a, int b, int n){
        if (n == 1 || n == 2)        return 1;    else        return (a*f(a, b, n - 1) + b*f(a, b, n - 2))%7;}int  main( ){
        int a, b, n;    cin >> a >> b >> n;    while (a!=0 || b!=0 || n!=0)    {
            n = n % 49;        cout << f(a, b, n)<
     
      > a >> b >> n;    }    return 0;}