Given an integer numRows, return the first numRows of Pascal's triangle.
In Pascal's triangle, each number is the sum of the two numbers directly above it.
Input: numRows = 5
Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]
Input: numRows = 1
Output: [[1]]
Input: numRows = 3
Output: [[1],[1,1],[1,2,1]]