leetcode[119]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?

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