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?

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class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> vec;
		if(rowIndex < 0) return vec;
		int prev = 0;

		for(int i=0; i <= rowIndex; i++) {
			if(i==0)
				vec.push_back(1);

			for(int j=vec.size()-1; j > 0; j--) {
				vec[j] = vec[j]+vec[j-1];
			}

			if(i > 0)
				vec.push_back(1);
		}

		return vec;
    }
};