LeetCode#118 Pascal's Triangle

Problem Definition:

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]
]


Solution:  关于杨辉三角

def generate(numRows):
        pt=[]
        for rn in range(numRows):
            row=[0]*(rn+1)
            row[0],row[rn]=1,1    
            for cl in range(1,rn):
                p1=pt[rn-1][cl-1]
                p2=pt[rn-1][cl]
                row[cl]=p1+p2
            pt+=[row]
        return pt