leetcode-118：杨辉三角

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

class Solution:
    def generate(self, numRows: int) -> List[List[int]]:
        ret = []
        for i in range(numRows):
            row = []
            for j in range(i+1):
                if j==0 or j==i:
                    row.append(1)
                else:
                    row.append(ret[i-1][j]+ret[i-1][j-1])
            ret.append(row)
        return ret