118. Pascal's Triangle

Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 5
Output:
[
     [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>> listAll = new ArrayList<>();
        if (numRows <= 0) return listAll;
        for (int i = 0; i < numRows; i++) {
            List<Integer> list = new ArrayList<>();
            list.add(1);
            if (i != 0) {
                List<Integer> prevList = listAll.get(i - 1);
                for (int j = 1; j < prevList.size(); j++) {
                    list.add(prevList.get(j-1) + prevList.get(j));
                }
                list.add(1);
            }
            listAll.add(list);
        }
        return listAll;   
    }
}