Climbing Stairs

class Solution {
public:
    int climbStairs(int n) {
        if(n==1)  return 1;
        if(n==2)  return 2;
        if(n<=0) return 0;
        vector<int> v;
        v.push_back(1);
        v.push_back(2);
        for(int i=3;i<=n;i++){
            v.push_back(v[i-3]+v[i-2]);
        }
        return v[n-1];
    }
};