[Array]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?

方法：在每一行的更新中，从后往前进行更新可以使代码更加简洁。

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