[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]
]

Solution:

public class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> ret=new ArrayList<List<Integer>>();
        if(numRows<1)
            return ret;
        List<Integer> al=new ArrayList<Integer>();
        al.add(1);
        ret.add(al);
        for(int i=1;i<numRows;++i){
            List<Integer> cur=new ArrayList<Integer>();
            cur.add(1);
            for(int j=1;j<al.size();++j){
                cur.add(al.get(j-1)+al.get(j));
            }
            cur.add(1);
            ret.add(cur);
            al=cur;
        }
        return ret;
    }
}