118杨辉三角

class Solution:
def generate(self, numRows: int) -> List[List[int]]:
res = [[] for i in range(numRows)]
for i in range(numRows):
res[i] = [1]*(i+1)
for j in range(1,i):
res[i][j] = res[i-1][j-1] + res[i-1][j]
return res