class Solution

class Solution {
    public int climbStairs(int n) {
        if (n <= 1) {
            return 1;
        }
        int prev = 1;
        int current = 1;
        for (int i=2; i<=n; ++i) {
            int next = prev + current;
            prev = current;
            current = next;
        }
        return current;
    }
}