f(n)=f(n-1)+f(n-2)+...+f(1)
f(n-1)=f(n-2)+...f(1)
得:f(n)=2*f(n-1)

public class Solution {
    public int JumpFloorII(int target) {
        return 1<<(target-1);
        //return (int)Math.pow(2,target-1);
    }
}