Spiral Matrix II
Given a positive integer   n , generate an   n x n   matrix   filled with elements from   1   to   n 2   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               Solution:         class Solution {        public int[][] generateMatrix(int n) {            int[][] matrix = new int[n][n];                 int top = 0, bottom = n - 1, left = 0, right = n     - 1;              int val = 1;            while (val <= n * n) {                     for (int i = left; i <= right;     i++)                            matrix[top][i] =     val++;                   top++;                      ...