Climbing Stairs

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?

简单递推

class Solution {public:    int climbStairs(int n) {        long long ans=0;        if(n==1) return 1;        if(n==2) return 2;        int a=1,b=2;        n-=2;        while(n--){            ans=a+b;            a=b;            b=ans;        }        return ans;    }};