[118. Pascal's Triangle](https://leetcode.com/problems/pascals-triangle/
Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.
Note: 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]
]
给一个正整数来生成一个帕斯卡三角形