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?

思路：

最开始的想法是用两个vector依次记录当前行和上一行的结果。但是要求用k的空间，所以使用一个vector，每次从后往前r[j] = r[j] + r[j-1]即可。

代码：

    vector<int> getRow(int rowIndex) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<int> result(rowIndex+1,0);
        result[0] = 1;
        int i, j;
        for(i = 1;  i <= rowIndex; i++){
            for(j = i; j > 0; j--){
                result[j] = result[j] + result[j-1];
            }
        }
        return result;
    }