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>> rec = new ArrayList<List<Integer>>(numRows);
        if(numRows <= 0)
          return rec;
        
        for(int i=0;i<numRows;i++)
         { 
          List<Integer> arr = new ArrayList<Integer>(i);
          for(int j=0;j<=i;j++)
          {   
              if(j==0)
                arr.add(1);
              else if(j>0&&j<i)
              {
                int t = rec.get(i-1).get(j-1) + rec.get(i-1).get(j);
                arr.add(t);
              }
              else
                arr.add(1);
                
          }
          rec.add(arr);
         }
        return rec;
    }
}