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?

杨辉三角，我们知道v[i][j] = v[i-1][j-1] + v[i-1][j]，于是，逆序相加法，可以防止值被覆盖，新的当前位置的值是当前位置的值加上当前位置之前一个位置的值, v[i] = v[i] + v[i-1]，这是参考的解法，我是没想到啦~~

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> vi(rowIndex + 1);
           vi[0] = 1;
        for (int i = 0; i <= rowIndex ; ++i)
        {
            for (int j = i; j > 0; --j)
            {
                vi[j] = vi[j] + vi[j-1];
            }
        }
        return vi;
    }
};