Q118 Pascal's Triangle

For example, given numRows = 5,
Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

class Solution:
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        if numRows == 0:
            return []
        rlist = []
        rlist.append([1])  # 第一行元素
        i = 1
        while i < numRows:
            rlist.append([])  # 新增下一行
            rlist[i].append(1)
            j = 0
            while j < len(rlist[i-1]) - 1:  # 构造下一行元素
                rlist[i].append(rlist[i-1][j] + rlist[i-1][j+1])
                j += 1
            rlist[i].append(1)
            i += 1
        return rlist

a = 5
b = Solution()
print(b.generate(a))  # [[1], [1, 1], [1, 2, 1], [1, 3, 3, 1], [1, 4, 6, 4, 1]]