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].

• Time: O(n)
• Space: O(1)

public List<Integer> getRow(int rowIndex) {
    List<Integer> row = null, pre = null;
    for (int i = 0; i < rowIndex + 1; i++) {
        row = new ArrayList<>();
        for (int j = 0; j <= i; j++) {
            if (j == 0 || j == i) {
                row.add(1);
            } else {
                row.add(pre.get(j - 1) + pre.get(j));
            }
        }
        pre = row;
    }
    return row;
}