Pascal's Triangle

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

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

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