119 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 getRow(int rowIndex) {
        vector res=vector(rowIndex+1,1);
        for (int i=2;i<=rowIndex;i++)
        {
            for (int j=i-1;j>0;j--)
            {
                res[j]=res[j]+res[j-1];
            }
        }
        return res;
    }
};