递推 + 大数 HDU1297

 1 import java.math.*;
 2 import java.util.*;
 3 
 4 public class Main
 5 {
 6     static BigInteger arr[] = new BigInteger [1000000];
 7     public static void main(String[] args)
 8     {
 9         Scanner cin=new Scanner(System.in);
10         int n;
11         while(cin.hasNext())
12         {
13             n=cin.nextInt();
14             arr[0]=BigInteger.valueOf(1);
15             arr[1]=BigInteger.valueOf(1);
16             arr[2]=BigInteger.valueOf(2);
17             arr[3]=BigInteger.valueOf(4);
18             arr[4]=BigInteger.valueOf(7);
19             for(int i=5;i<=n;i++)
20             {
21                 arr[i]=BigInteger.ZERO;
22                 arr[i]=arr[i].add(arr[i-1]);
23                 arr[i]=arr[i].add(arr[i-2]);
24                 arr[i]=arr[i].add(arr[i-4]);
25             }
26             System.out.println(arr[n]);
27         }
28     }
29 }