Leetcode: Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n^2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

跟Spiral Matrix一样的处理，甚至还简单一些，这个确定是方阵。分层，然后按照上右下左的顺序放入数组中。每个元素只访问一次，时间复杂度是O(n^2)。

public class Solution {
    public int[][] generateMatrix(int n) {
        int[][] res = new int[n][n];
        if (n < 0) return null;
        int levelNum = n / 2;
        int count = 1;
        for (int level=0; level<levelNum; level++) {
            for (int i=level; i<n-1-level; i++) {
                res[level][i] = count++;
            }
            for (int i=level; i<n-1-level; i++) {
                res[i][n-1-level] = count++;
            }
            for (int i=n-1-level; i>level; i--) {
                res[n-1-level][i] = count++;
            }
            for (int i=n-1-level; i>level; i--) {
                res[i][level] = count++;
            }
        }
        if (n % 2 == 1) {
            res[levelNum][levelNum] = count;
        }
        return res;
    }
}