题目链接：

设 f[i][j] 表示第i天年龄为j的母牛个数，其中j=4代表所有年龄达到4岁的成年母牛，则：

• f[1][4] = 1
• f[1][i] = 0, i = 1,2,3
• f[i][4] = f[i-1][4] + f[i-1][3]
• f[i][3] = f[i-1][2]
• f[i][2] = f[i-1][1]
• f[i][1] = f[i][4]

代码：

#include 
    
     #include 
     
      using namespace std;const int maxn = 56;long long f[maxn][5];void init() {    f[1][4] = 1;    for (int i = 2; i < maxn; i ++) {        f[i][4] = f[i-1][4] + f[i-1][3];        f[i][3] = f[i-1][2];        f[i][2] = f[i-1][1];        f[i][1] = f[i][4];    }}void output(int i) {    cout << f[i][1] + f[i][2] + f[i][3] + f[i][4] << endl;}int n;int main() {    init();    //for (int i = 1; i <= 10; i ++) output(i);    while (cin >> n) {        if (!n) break;        output(n);    }    return 0;}