<p>You are given an <code>m x n</code> binary grid <code>grid</code> where <code>1</code> represents land and <code>0</code> represents water. An <strong>island</strong> is a maximal <strong>4-directionally</strong> (horizontal or vertical) connected group of <code>1</code>&#39;s.</p>

<p>The grid is said to be <strong>connected</strong> if we have <strong>exactly one island</strong>, otherwise is said <strong>disconnected</strong>.</p>

<p>In one day, we are allowed to change <strong>any </strong>single land cell <code>(1)</code> into a water cell <code>(0)</code>.</p>

<p>Return <em>the minimum number of days to disconnect the grid</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/24/land1.jpg" style="width: 500px; height: 169px;" />
<pre>
<strong>Input:</strong> grid = [[0,1,1,0],[0,1,1,0],[0,0,0,0]]

<strong>Output:</strong> 2
<strong>Explanation:</strong> We need at least 2 days to get a disconnected grid.
Change land grid[1][1] and grid[0][2] to water and get 2 disconnected island.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/24/land2.jpg" style="width: 404px; height: 85px;" />
<pre>
<strong>Input:</strong> grid = [[1,1]]
<strong>Output:</strong> 2
<strong>Explanation:</strong> Grid of full water is also disconnected ([[1,1]] -&gt; [[0,0]]), 0 islands.
</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;= 30</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

class Solution {
public:
    typedef struct Coor {
        int r;
        int c;
    } Coor;

    int reachLandCount(vector<vector<int>>& grid, int r, int c) {
        int m = grid.size();
        int n = grid[0].size();
        vector<vector<int>> visited(m, vector<int>(n));
        vector<Coor> trav;
        trav.emplace_back(r, c);
        auto addToTrav = [&m, &n, &grid, &visited, &trav](int x, int y) {
            if ((x >= 0 && x < m && y >= 0 && y < n) == false) return;
            if (grid[x][y] == 0) return;
            if (visited[x][y]) return;
            trav.emplace_back(x, y);
        };
        int landCnt = 0;
        while (trav.size()) {
            int r = trav.back().r;
            int c = trav.back().c;
            trav.pop_back();
            if (visited[r][c]) continue;
            visited[r][c] = 1;
            ++landCnt;
            addToTrav(r + 1, c);
            addToTrav(r - 1, c);
            addToTrav(r, c + 1);
            addToTrav(r, c - 1);
        }
        return landCnt;
    }

    int minDays(vector<vector<int>>& grid) {
        int m = grid.size();
        int n = grid[0].size();
        vector<Coor> lands;
        for (int r = 0; r < m; ++r)
            for (int c = 0; c < n; ++c)
                if (grid[r][c] == 1) lands.emplace_back(r, c);

        if (lands.empty()) return 0;
        if (lands.size() != reachLandCount(grid, lands.back().r, lands.back().c)) return 0;
        if (lands.size() == 1) return 1;

        auto isLand = [&m, &n, &grid](int x, int y) {
            if (!(x >= 0 && x < m && y >= 0 && y < n)) return false;
            return grid[x][y] == 1 ? true : false;
        };
        for (auto &cur: lands) {
            int r = cur.r;
            int c = cur.c;
            if (grid[r][c] == 0) continue;
            grid[r][c] = 0;
            if (isLand(r + 1, c)) {
                if (reachLandCount(grid, r + 1, c) != lands.size() - 1) return 1;
            } else if (isLand(r - 1, c)) {
                if (reachLandCount(grid, r - 1, c) != lands.size() - 1) return 1;
            } else if (isLand(r, c + 1)) { 
                if (reachLandCount(grid, r, c + 1) != lands.size() - 1) return 1;
            } else if (isLand(r, c - 1)) { 
                if (reachLandCount(grid, r, c - 1) != lands.size() - 1) return 1;
            }
            grid[r][c] = 1;
        }
        return 2;
    }
};
52460