class Solution {
    public int climbStairs(int n) {
        if (n==1 || n==2)
            return n;
        int a = 1, b = 2, c = 3, i = 3;
        while (i < n) {
            int tmp = c;
            c = b+c;
            a = b;
            b = tmp;
            i++;
        }
        return c;
    }
}