Pascal's Triangle II

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?

class Solution {
public:
vector<int> getRow(int rowIndex) {
vector<int>res;
if(rowIndex<0)
return res;
res.push_back(1);
for(int i=2;i<= rowIndex+1;i++)
{
int pre=1;
for(int j=1;j<i-1;j++)
{
int tmp=res[j];
res[j]+=pre;
pre=tmp;
}
res.push_back(1);
}
return res;
}
};