118. Pascal's Triangle

time: O(n^2)

space: O(n)

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        for (int i = 0 ; i < numRows; i++) {
            list.add(0, 1);
            for (int j = 1; j < list.size() - 1; j++) {
                list.set(j, list.get(j) + list.get(j+1));
            }
            res.add(new ArrayList<>(list));
        }
        return res;
    }
}