Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.For example,Given n = 3,You should return the following matrix:[ [ 1, 2, 3 ], [ 8, 9, 4 ], [ 7, 6, 5 ]]

 1 public class Solution { 2     public int[][] generateMatrix(int n) { 3         int[][] res = new int[n][n]; 4         if (n < 0) return null; 5         int levelNum = n / 2; 6         int count = 1; 7         for (int level=0; level<levelNum; level++) { 8             for (int i=level; i<n-1-level; i++) { 9                 res[level][i] = count++;10             }11             for (int i=level; i<n-1-level; i++) {12                 res[i][n-1-level] = count++;13             }14             for (int i=n-1-level; i>level; i--) {15                 res[n-1-level][i] = count++;16             }17             for (int i=n-1-level; i>level; i--) {18                 res[i][level] = count++;19             }20         }21         if (n % 2 == 1) {22             res[levelNum][levelNum] = count;23         }24         return res;25     }26 }