算法：兔子生兔子

 1 public long sumRubbit1(int n){
 2         long total = 0;    //兔子总数
 3         int big = 1;    //大兔子总数
 4         int middle = 0;    //中兔子总数
 5         int little = 0;    //小兔子总数
 6         if(n==1){
 7             total = big;
 8         }else if(n==2){
 9             little = big;
10             total = big+little;
11         }else{
12             little = big;
13             int beforMiddle =0;    //上个月的中兔子总数
14             for(int i=3;i<=n;i++){
15                 beforMiddle = middle;
16                 middle = little;        //中兔子数=上月小兔子数
17                 little = big;            //小兔子数=上月大兔子数
18                 big = big +beforMiddle;    //大兔子数=上月大兔子数+上月中兔子数
19                 total = big+middle+little;
20             }
21         }
22         return total;
23     }

 1 public long sumRubbit2(int n){
 2         long total = 0;    //兔子总数
 3         if(n==1){
 4             total = 1;
 5         }else if(n==2){
 6             total = 2;
 7         }else if(n==3){
 8             total = 3;
 9         }else{
10             total = sumRubbit2(n-1)+sumRubbit2(n-3);
11         }
12         return total;
13     }