leetcode_question_119 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?

vector<int> getRow(int rowIndex) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> matrix(rowIndex+1);
        if(rowIndex < 0) return matrix;
        for(int i = 0; i <= rowIndex; ++i)
        {
    		if( i == 0) {matrix[0] = 1;continue;}
			int mid = i/2;
			int tmpnum = matrix[0];
			for(int j = 1; j <= mid; ++j)
			{
				int tmp = tmpnum + matrix[j];
				tmpnum = matrix[j];
				matrix[j] = tmp;
			}
			++mid;
			while(mid <= i){
				matrix[mid] = matrix[i-mid];
				++mid;
			}
        }
        return matrix;
    }