<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:
    bool isSafe(int i, int j, vector<vector<int>>& grid, vector<vector<bool>>& vis, int rows, int cols) {
        return i >= 0 && j >= 0 && i < rows && j < cols && grid[i][j] == 1 && !vis[i][j];
    }

    void dfs(int i, int j, vector<vector<int>>& grid, vector<vector<bool>>& vis) {
        vis[i][j] = true;
        int rows = grid.size(), cols = grid[0].size();
        int ROWS[4] = {0, -1, 0, 1};
        int COLS[4] = {-1, 0, 1, 0};
        for (int k = 0; k < 4; k++) {
            int I = i + ROWS[k];
            int J = j + COLS[k];
            if (isSafe(I, J, grid, vis, rows, cols)) dfs(I, J, grid, vis);
        }
    }

    int numEnclaves(vector<vector<int>>& grid) {
        int rows = grid.size(), cols = grid[0].size();
        vector<vector<bool>> vis(rows, vector<bool> (cols, false));
        // start traversing from the boundary cells
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (i != 0 && j != 0 && i != rows - 1 && j != cols - 1) continue;
                if (grid[i][j] == 1 && !vis[i][j]) dfs(i, j, grid, vis);
            }
        }
        int ans = 0;
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (grid[i][j] == 1 && !vis[i][j]) ans++;
            }
        }
        return ans;
    }
};
42892