leetcode #119 in cpp

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

Code:

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> res;
        if(rowIndex < 0) return res;
        res.push_back(1);
        rowIndex --; 
        while(rowIndex >= 0){
            res.insert(res.begin(), 1);
            for(int i = 1; i < res.size(); i ++){
                if(i+1 < res.size()) res[i] = res[i] + res[i+1];
            }
            rowIndex --;
        }
        return res;
    }
};