118. 杨辉三角

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> rtnList = new ArrayList<List<Integer>>();
        List<Integer> list = new ArrayList<Integer>();
        for (int i = 1; i <= numRows; i++) {
            list.add(1);
            for (int j = i - 2; j > 0; j--) {
                list.set(j, list.get(j) + list.get(j - 1));
            }
            rtnList.add(new ArrayList<Integer>(list));
        }
        return rtnList;
    }