Leetcode: 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 ArrayList<ArrayList<Integer>> generate(int numRows) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        List<Integer> item;
        if (numRows <= 0) return result;
        for (int i=0; i<numRows; i++) {
            item = new ArrayList<Integer>();
            for (int j=0; j<=i; j++) {
                if (j==0 || j==i) item.add(1);
                else {
                    List<Integer> prev = result.get(i-1);
                    item.add(prev.get(j-1) + prev.get(j));
                }
            }
            result.add(new ArrayList<Integer>(item));
        }
        return result;
    }
}