Number of Enclaves
<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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1020.Number%20of%20Enclaves/images/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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1020.Number%20of%20Enclaves/images/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>
Depth-First Search; Breadth-First Search; Union Find; Array; Matrix
impl Solution {
    pub fn num_enclaves(mut grid: Vec<Vec<i32>>) -> i32 {
        let m = grid.len();
        let n = grid[0].len();
        let dirs = [-1, 0, 1, 0, -1];

        fn dfs(grid: &mut Vec<Vec<i32>>, i: usize, j: usize, dirs: &[i32; 5]) {
            grid[i][j] = 0;
            for k in 0..4 {
                let (x, y) = (i as i32 + dirs[k], j as i32 + dirs[k + 1]);
                if let Some(row) = grid.get_mut(x as usize) {
                    if let Some(&mut 1) = row.get_mut(y as usize) {
                        dfs(grid, x as usize, y as usize, dirs);
                    }
                }
            }
        }

        for j in 0..n {
            if grid[0][j] == 1 {
                dfs(&mut grid, 0, j, &dirs);
            }
            if grid[m - 1][j] == 1 {
                dfs(&mut grid, m - 1, j, &dirs);
            }
        }

        for i in 0..m {
            if grid[i][0] == 1 {
                dfs(&mut grid, i, 0, &dirs);
            }
            if grid[i][n - 1] == 1 {
                dfs(&mut grid, i, n - 1, &dirs);
            }
        }

        grid.into_iter().flatten().filter(|&x| x == 1).count() as i32
    }
}
