leetcode118 Pascal's Triangle

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

public class leetcode118 {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        for (int n = 1; n <= numRows; n++) {
            List<Integer> temp = new ArrayList<>();
            int num = 1;
            for (int k = 1; k <= n; k++) {
                if (k == 1)
                    num = 1;
                else
                    num = num * (n - k+1) / (k - 1);
                temp.add(num);
            }
            res.add(temp);
        }
        return res;
    }
}