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?

Solution: from back to forth...

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> res(rowIndex+1, 1);
        for(int i = 1; i <= rowIndex; i++) {
            for(int j = i-1; j >= 1; j--) {
                res[j] += res[j-1]; 
            }
        }
        return res;
    }
};