GivennumRows, generate the firstnumRowsof Pascal's triangle.

For example, givennumRows= 5,
Return

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

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> ans = new ArrayList<>();
        if (numRows == 0) return ans;
        ans.add(new ArrayList<>(Arrays.asList(1)));
        if (numRows == 1) return ans;

        for (int i = 1; i < numRows; i++){
            List<Integer> temp = new ArrayList<>();
            for (int j = 0; j <= i; j++){
                int v1 = j < ans.get(i - 1).size() ? ans.get(i - 1).get(j) : 0;
                int v2 = j >= 1 ? ans.get(i - 1).get(j - 1) : 0;
                int val = v1 + v2;
                temp.add(val);
            }
            ans.add(temp);
        }

        return ans;
    }
}