1 class Solution {
2 List<List<Integer>> res = new ArrayList<>();
3 public List<List<Integer>> generate(int numRows) {
4 helper(1, numRows, new ArrayList<>(), null);
5 return res;
6
7 }
8
9 public void helper(int num, int total, List<Integer> list, List<Integer> prev) {
10 if(num > total) return;
11
12 for(int i = 1; i <= num; i++) {
13 if(num == 1 || num == 2) {
14 list.add(1);
15 }else {
16 if(i == 1 || i == num) {
17 list.add(1);
18 }else {
19 list.add(prev.get(i-2) + prev.get(i-1));//注意是i-2 循环用的是1开始
20 }
21 }
22 }
23 res.add(list);
24 helper(num+1, total, new ArrayList<>(), list);
25 }
26 }
