Pascal's Triangle

Given numRows, generate the first numRows of 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(object):
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        rows = []
        for i in range(numRows):
            rows += [1],
            if i == 0:
                continue
            else:
                rows[-1] += [ rows[-2][x] + rows[-2][x - 1] for x in range(1, i)]
                rows[-1] += 1,
        return rows

Trick:

class Solution(object):
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        res = [[1]]
        for i in range(1, numRows):
            res += map(lambda x, y: x + y, res[-1] + [0], [0] + res[-1]),
        return res[:numRows] #  the same as if not numsRows: return []

Explanation: Any row can be constructed using the offset sum of the previous row. Example:

    1 3 3 1 0 
 +  0 1 3 3 1
 =  1 4 6 4 1