class Solution:
def generate(self, numRows: int) -> List[List[int]]:
res = [[1], [1,1]]
if numRows < 3: #前两层特殊
return res[:numRows]
row = [1,1]
#从第三层开始，一层层迭代
for i in range(3, numRows+1):
new_row = [1] * i #新层初始全1
for j in range(1, i-1):
new_row[j] = row[j-1] + row[j]
row = new_row #替换旧层
res.append(new_row)
return res
