Spiral Matrix II leetcode java

題目：

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 ]
]


題解：
這道題跟Spiral Matrix想法也是類似的，就是依照矩陣從外圈到內圈建立。
要考慮如果是奇數行列的話，最中心的那個點要單加。

程式碼如下：

 1     public int[][] generateMatrix(int n) {
 2         int[][] res = new int[n][n];
 3         int k = 1;
 4         int top = 0, bottom = n - 1, left = 0, right = n - 1;
 5         while (left < right && top < bottom) {
 6             for (int j = left; j < right; j++) {
 7                 res[top][j] = k++;
 8             }
 9             for (int i = top; i < bottom; i++) {
10                 res[i][right] = k++;
11             }
12             for (int j = right; j > left; j--) {
13                 res[bottom][j] = k++;
14             }
15             for (int i = bottom; i > top; i--) {
16                 res[i][left] = k++;
17             }
18             left++;
19             right--;
20             top++;
21             bottom--;
22         }
23         if (n % 2 != 0)
24             res[n / 2][n / 2] = k;
25         return res;
26     }

Reference：leetcodenotes.wordpress.com/2013/11/23/leetcode-spiral-matrix-%E6%8A%8A%E4%B8%80%E4%B8%AA2d-matrix%E7%94%A8%E8%9E%BA%E6%97%8B%E6%96%B9%E5%BC%8F%E6%89%93%E5%8D%B0/