[LeetCode] 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?

class Solution {
public:
    int climbStairs(int n) {
        int n_1 = 1, n_2 = 1;
        int res = 1;
        for(int i=2;i<=n;i++)
        {
            res = n_1 + n_2;
            n_2 = n_1;
            n_1 = res;
        }
        return res;
    }
};