Pascal's Triangle II

Given an index k, return the k^th 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：从后面往前加

class Solution {public:    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> row;                for(int i=0; i<=rowIndex; i++){            for(int j=row.size()-1; j>0; j--){                row[j] = row[j] + row[j-1];            }                row.push_back(1);        }        return row;    }};