杭电1297-用string

#include <iostream>#include <cstring>using namespace std;string Add(string a,string b){int i,j,sum=0;if(a.length()<b.length())swap(a,b);for(i=a.length()-1,j=b.length()-1;i>=0;i--,j--){sum+=a[i]-'0';if(j>=0)sum+=b[j]-'0';a[i]=sum%10+'0';sum/=10;}if(sum)a='1'+a;return a;}int main(){    int a,j,n;    string f[2000];    f[0]="1";    f[1]="1";    f[2]="2";    f[3]="4";    for(j=4;j<=1000;j++)    {        f[j]=Add(Add(f[j-1],f[j-2]),f[j-4]);    }    while(cin>>n)    {        cout<<f[n]<<endl;    }return 0;}