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;
}
};