leetcode Pascal's Triangle 2.22 难度系数2

[
[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>> result = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> ini = new ArrayList<>();
if (numRows == 0) {
return result;
}
ini.add(1);
result.add(ini);
for (int i = 1; i < numRows; i++) {
ArrayList<Integer> previous = result.get(i - 1);
ArrayList<Integer> current = new ArrayList<Integer>();
current.add(1);
for (int j = 1; j < i; j++) {
int tmp = previous.get(j - 1) + previous.get(j);
current.add(tmp);
}
current.add(1);
result.add(current);
}
return result;

}
}