<p>You are given an <code>m x n</code> binary matrix <code>grid</code>. An island is a group of <code>1</code>&#39;s (representing land) connected <strong>4-directionally</strong> (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.</p>

<p>The <strong>area</strong> of an island is the number of cells with a value <code>1</code> in the island.</p>

<p>Return <em>the maximum <strong>area</strong> of an island in </em><code>grid</code>. If there is no island, return <code>0</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/05/01/maxarea1-grid.jpg" style="width: 500px; height: 310px;" />
<pre>
<strong>Input:</strong> grid = [[0,0,1,0,0,0,0,1,0,0,0,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,1,1,0,1,0,0,0,0,0,0,0,0],[0,1,0,0,1,1,0,0,1,0,1,0,0],[0,1,0,0,1,1,0,0,1,1,1,0,0],[0,0,0,0,0,0,0,0,0,0,1,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,0,0,0,0,0,0,1,1,0,0,0,0]]
<strong>Output:</strong> 6
<strong>Explanation:</strong> The answer is not 11, because the island must be connected 4-directionally.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> grid = [[0,0,0,0,0,0,0,0]]
<strong>Output:</strong> 0
</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;= 50</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

struct pair_hash {
    template <class T1, class T2>
    size_t operator() (const pair<T1, T2> &pair) const {
        return hash<T1>()(pair.first) ^ hash<T2>()(pair.second);
    }
};

class Solution {
public:
    unordered_set<pair<int, int>, pair_hash> visited;

    int findArea(int i, int j, vector<vector<int>>& grid) {
        if (0 <= i && i < grid.size() && 0 <= j && j < grid[0].size() && 
            visited.find({i, j}) == visited.end() && grid[i][j] == 1) {
            visited.insert({i, j});
            // recursively call the function for all four directions
            return 1 + findArea(i - 1, j, grid) 
                     + findArea(i + 1, j, grid) 
                     + findArea(i, j - 1, grid) 
                     + findArea(i, j + 1, grid);
        }
        return 0;
    }

    int maxAreaOfIsland(vector<vector<int>>& grid) {
        int result = 0;

        for (int i = 0; i < grid.size(); ++i) {
            for (int j = 0; j < grid[0].size(); ++j) {
                result = max(result, findArea(i, j, grid));
            }
        }

        return result;
    }
};
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