118. 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>> rst = new ArrayList<ArrayList<Integer>>();
        
        int i=0, j=0;
        while(i<numRows){
            ArrayList<Integer> list = new ArrayList<Integer>();
                j=0;
                while(j<=i){
                    int item;
                    if(j==0 || j==i) item = 1;
                    else item = rst.get(i-1).get(j-1) + rst.get(i-1).get(j);
                    list.add(item);
                    j++;
                }
            i++;
            rst.add(list);
        }
        
        return rst;
    }
}