Pascal ' s Triangle

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

For example, given numrows = 5,
Return

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

Each layer of Pascal starts and ends at 1 and from the second position its value is the diagonal two number of its upper layer, and with a for loop you can step through this construction code as follows:
  
public class Solution {public    list<list<integer>> generate (int numrows) {        list<list< Integer>> res=new arraylist<list<integer>> ();        if (numrows==0) return res;        List<integer> tmp=new arraylist<integer> ();        Tmp.add (1);        Res.add (TMP);        if (numrows==1) return res;         for (int i=1;i<numrows;i++) {            list<integer> ntmp=new arraylist<integer> ();            Ntmp.add (1);            for (int j=0;j<i-1;j++) {                ntmp.add (Res.get (i-1). Get (j) +res.get (i-1). Get (J+1));            }            Ntmp.add (1);            Res.add (ntmp);        }        return res;    }}

Pascal ' s Triangle