Input: 5

Output:

[

     [1],

    [1,1],

   [1,2,1],

  [1,3,3,1],

 [1,4,6,4,1]

]

class Solution(object):

    def generate(self, numRows):

        """

        :type numRows: int

        :rtype: List[List[int]]

        """

        res = []

        for i in range(numRows):

            if i == 0:

                row = [1]

            else:

                row = [1]

                for j in range(1, i):

                    row.append(res[-1][j] + res[-1][j-1])

                row.append(1)

            res.append(row)

        return res