Spiral Matrix II
<p>Given a positive integer <code>n</code>, generate an <code>n x n</code> <code>matrix</code> filled with elements from <code>1</code> to <code>n<sup>2</sup></code> in spiral order.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0059.Spiral%20Matrix%20II/images/spiraln.jpg" style="width: 242px; height: 242px;" />
<pre>
<strong>Input:</strong> n = 3
<strong>Output:</strong> [[1,2,3],[8,9,4],[7,6,5]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> [[1]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 20</code></li>
</ul>
Array; Matrix; Simulation
class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> ans(n, vector<int>(n, 0));
        const int dirs[5] = {0, 1, 0, -1, 0};
        int i = 0, j = 0, k = 0;
        for (int v = 1; v <= n * n; ++v) {
            ans[i][j] = v;
            int x = i + dirs[k], y = j + dirs[k + 1];
            if (x < 0 || x >= n || y < 0 || y >= n || ans[x][y] != 0) {
                k = (k + 1) % 4;
            }
            i += dirs[k];
            j += dirs[k + 1];
        }
        return ans;
    }
};
