<p>You are given an <code>m x n</code> binary matrix <code>grid</code>, where <code>0</code> represents a sea cell and <code>1</code> represents a land cell.</p>

<p>A <strong>move</strong> consists of walking from one land cell to another adjacent (<strong>4-directionally</strong>) land cell or walking off the boundary of the <code>grid</code>.</p>

<p>Return <em>the number of land cells in</em> <code>grid</code> <em>for which we cannot walk off the boundary of the grid in any number of <strong>moves</strong></em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/18/enclaves1.jpg" style="width: 333px; height: 333px;" />
<pre>
<strong>Input:</strong> grid = [[0,0,0,0],[1,0,1,0],[0,1,1,0],[0,0,0,0]]
<strong>Output:</strong> 3
<strong>Explanation:</strong> There are three 1s that are enclosed by 0s, and one 1 that is not enclosed because its on the boundary.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/18/enclaves2.jpg" style="width: 333px; height: 333px;" />
<pre>
<strong>Input:</strong> grid = [[0,1,1,0],[0,0,1,0],[0,0,1,0],[0,0,0,0]]
<strong>Output:</strong> 0
<strong>Explanation:</strong> All 1s are either on the boundary or can reach the boundary.
</pre>

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

<ul>
	<li><code>m == grid.length</code></li>
	<li><code>n == grid[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 500</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

class Solution {
   public:
    const static int N = 500;
    bool vis[N][N];
    vector<pair<int, int>> fr = {{0, 1}, {0, -1}, {-1, 0}, {1, 0}};

    int n, m, cnt = 0, tmp;
    bool corner;
    vector<vector<int>> v;

    bool valid(int i, int j) {
        return !(i < 0 || i >= n || j < 0 || j >= m || v[i][j] == 0 || vis[i][j] == true);
    }

    bool is_corner(int i, int j) {
        return (i == 0 || i == n - 1 || j == 0 || j == m - 1);
    }

    void dfs(int si, int sj) {
        vis[si][sj] = true;
        corner = corner || is_corner(si, sj);
        tmp++;

        for (int i = 0; i < 4; i++) {
            int ci = si + fr[i].first;
            int cj = sj + fr[i].second;
            if (valid(ci, cj)) dfs(ci, cj);
        }
    }

    int numEnclaves(vector<vector<int>> &grid) {
        memset(vis, false, sizeof(vis));
        v = grid;
        n = v.size(), m = v.front().size();

        for (int i = 0; i < n; i++)
            for (int j = 0; j < m; j++) {
                if (valid(i, j)) {
                    corner = false;
                    tmp = 0;
                    dfs(i, j);
                    if (!corner)
                        cnt += tmp;
                }
            }

        return cnt;
    }
};
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