思路：
因为n级台阶，第一步有n种跳法：跳1级、跳2级、到跳n级
跳1级，剩下n-1级，则剩下跳法是f(n-1)
跳2级，剩下n-2级，则剩下跳法是f(n-2)
所以f(n)=f(n-1)+f(n-2)+...+f(1)
因为f(n-1)=f(n-2)+f(n-3)+...+f(1)
function jumpFloorII(number){
// write code here
if(number == 1) {
return 1
}
let fn = 1
let f = 1
for(let i =2; i <= number; i++){
fn = 2*f
f = fn
}
return fn
}
原文：https://www.cnblogs.com/lyt0207/p/12522716.html