LeetCode＃118. Yanghui triangle

subject :

Given a nonnegative integer numRows, Generate 「 Yang hui triangle 」 Before numRows That's ok .

stay 「 Yang hui triangle 」 in , Each number is the sum of the numbers at the top left and right of it .

Example 1:

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

Input : numRows = 1
Output : [[1]]

source ： Power button （LeetCode）
link ：https://leetcode-cn.com/problems/pascals-triangle
 

The problem is not difficult , We can find out the special situation first ( When numsRows be equal to 1 or 2 The case when ), Then find out the rules and operate according to the animation demonstration .

class Solution:
    def generate(self, numRows: int) -> List[List[int]]:
        if numRows==1:
            return [[1]]
        elif numRows==2:
            return [[1],[1,1]]
        else:
            end=[[1],[1,1]]
            j=1
            while j <= numRows-2:
                ls=[1]
                for i in range(1,j+1):
                    ls.append(end[j][i-1]+end[j][i])
                ls.append(1)
                end.append(ls)
                j+=1
        return end