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

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