leetcode: 118. Pascal's Triangle

Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

class Solution():
    def generate(self, n):
        if n == 0:
            return []
        if n == 1:
            return [[1]]
        else:
            res = self.generate(n-1)
            level = list(map(lambda x, y: x+y, [0]+res[-1], res[-1]+[0]))
            return res + [level]