https://leetcode.com/problems/spiral-matrix-ii/
Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.
Example 1:
Input: n = 3 Output: [[1,2,3],[8,9,4],[7,6,5]] Example 2:
Input: n = 1 Output: [[1]]
Constraints:
1 <= n <= 20