public:
int jumpFloorII(int number) {
long long f[40],sum=0;
f[1]=1;f[2]=2;
for(int i=3;i<=20;i++){
sum=0;
for(int j=1;j<i;j++){
sum+=f[j];//递推
}
f[i]=sum+1;
}
return f[number];
}
};