Pascal's Triangle

class Solution:    # @param {integer} numRows    # @return {integer[][]}    def generate(self, numRows):        tri=[]        if numRows==0:            return []        cur=[1]        tri.append(cur)        for i in range(1,numRows):            pre=cur            cur=[1]            for j in range(1,i):                cur.append(pre[j-1]+pre[j])            cur.append(1)            tri.append(cur)        return tri