<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:
    void dfs(vector<vector<int>>& board, int row, int col, vector<vector<int>>& vis) {
        if (row - 1 >= 0 && !vis[row - 1][col] && board[row - 1][col] == 1) {
            vis[row - 1][col] = 1;
            dfs(board, row - 1, col, vis);
        }
        if (row + 1 < board.size() && !vis[row + 1][col] && board[row + 1][col] == 1) {
            vis[row + 1][col] = 1;
            dfs(board, row + 1, col, vis);
        }
        if (col - 1 >= 0 && !vis[row][col - 1] && board[row][col - 1] == 1) {
            vis[row][col - 1] = 1;
            dfs(board, row, col - 1, vis);
        }
        if (col + 1 < board[0].size() && !vis[row][col + 1] && board[row][col + 1] == 1) {
            vis[row][col + 1] = 1;
            dfs(board, row, col + 1, vis);
        }
    }
    int numEnclaves(vector<vector<int>>& board) {
        vector<vector<int>> visited(board.size(), vector<int>(board[0].size(), 0));
        int res = 0;
        for (int i = 0; i < board.size(); i++) {
            if (board[i][0] == 1) {
                visited[i][0] = 1;
                dfs(board, i, 0, visited);
            }
        }
        for (int i = 0; i < board[0].size(); i++) {
            if (board[board.size() - 1][i] == 1) {
                visited[board.size() - 1][i] = 1;
                dfs(board, board.size() - 1, i, visited);
            }
        }
        for (int i = 0; i < board[0].size(); i++) {
            if (board[0][i] == 1) {
                visited[0][i] = 1;
                dfs(board, 0, i, visited);
            }
        }
        for (int i = 0; i < board.size(); i++) {
            if (board[i][board[0].size() - 1] == 1) {
                visited[i][board[0].size() - 1] = 1;
                dfs(board, i, board[0].size() - 1, visited);
            }
        }
        for (int i = 1; i < visited.size() - 1; i++) {
            for (int j = 1; j < visited[0].size() - 1; j++) {                
                if (!visited[i][j] && board[i][j] == 1) {
                    res++;
                }
            }
        }
        return res;
    }
};
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