You are climbing a stair case. It takes n steps to reach to the top.
Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?
Note: Given n will be a positive integer.
每次爬1到2步有多少種方法可以爬上n階臺(tái)階
思路:
n=1時(shí) 有1種方法
n=2時(shí)，有兩種方法
n=3時(shí),有先爬到臺(tái)階2再爬1步(此處有f(2)種方法，最后一步唯一且確定)，與先爬到臺(tái)階1再爬2步（此處有f(1)種方法，最后一步唯一且確定）兩種策略，共f(2)+f(1)
n=4時(shí)，有先爬到臺(tái)階3再爬1步，與先爬到臺(tái)階2再爬2步兩種策略，共f(3)+f(2)
n=5時(shí)，.....
即有f(n)=f(n-1)+f(n-2)

var climbStairs = function(n) {
    var res = []
    res[1] = 1
    res[2] = 2 
    for(var i =3; i<n; i++){
        res[i] = res [i-1] + res [i-2]
    }
    return res[n]
};