# 0059. Spiral Matrix II

<https://leetcode.com/problems/spiral-matrix-ii>

## Description

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`

## ac

```java
// no special knowledge, just spacial imagination, be careful, writing codes by looking at sample

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] res = new int[n][n];
        // corner case
        if (n == 0) return res;

        // rb, re, cb, ce
        int rb = 0, re = n - 1, cb = 0, ce = n - 1;
        int num = 1;
        while (rb <= re) {
            // final one
            if (rb == re) res[rb][rb] = num;
            // top
            for (int c = cb; c < ce; c++) {
                res[rb][c] = num;
                num++;
            }
            // right
            for (int r = rb; r < re; r++) {
                res[r][ce] = num;
                num++;
            }
            // buttom
            for (int c = ce; c > cb; c--) {
                res[re][c] = num;
                num++;
            }
            // left
            for (int r = re ; r > rb; r--) {
                res[r][cb] = num;
                num++;
            }
            rb++;
            re--;
            cb++;
            ce--;
        }
        return res;
    }
}
```