climb stairs 1 or 2 stairs (pure recursion)

class Solution {
    public int climbStairs(int n) {
        if(n < 0){
            return 0;
        }
        if(n == 0 || n == 1){
            return 1;
        }
        if(n == 2){
            return 2;// 1+1 & 2
        }
        return climbStairs(n-2) + climbStairs(n-1);
    }

}