# 59. Spiral Matrix II (M)

Given a positive integer `n`, generate an `n x n` `matrix` filled with elements from `1` to `n2` in spiral order.

 

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/11/13/spiraln.jpg)

```
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:

有了上面的铺垫，稍微改一下代码即可完成这道题：

```java
int[][] generateMatrix(int n) {
    int[][] matrix = new int[n][n];
    int upper_bound = 0, lower_bound = n - 1;
    int left_bound = 0, right_bound = n - 1;
    // 需要填入矩阵的数字
    int num = 1;
    
    while (num <= n * n) {
        if (upper_bound <= lower_bound) {
            // 在顶部从左向右遍历
            for (int j = left_bound; j <= right_bound; j++) {
                matrix[upper_bound][j] = num++;
            }
            // 上边界下移
            upper_bound++;
        }
        
        if (left_bound <= right_bound) {
            // 在右侧从上向下遍历
            for (int i = upper_bound; i <= lower_bound; i++) {
                matrix[i][right_bound] = num++;
            }
            // 右边界左移
            right_bound--;
        }
        
        if (upper_bound <= lower_bound) {
            // 在底部从右向左遍历
            for (int j = right_bound; j >= left_bound; j--) {
                matrix[lower_bound][j] = num++;
            }
            // 下边界上移
            lower_bound--;
        }
        
        if (left_bound <= right_bound) {
            // 在左侧从下向上遍历
            for (int i = lower_bound; i >= upper_bound; i--) {
                matrix[i][left_bound] = num++;
            }
            // 左边界右移
            left_bound++;
        }
    }
    return matrix;
}
```

至此，两道螺旋矩阵的题目也解决了。