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?

编程要细心！

class Solution {public:    int climbStairs(int n) {        if( n == 1 || n == 2)            return n;        int first = 1, second = 2;        int res = 0;        for(int i = 3 ; i <= n ; ++i){            res = first + second;            first = second;            second = res;        }        return second;    }};