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;

    }