<p>Given an <code>n x n</code> binary matrix <code>grid</code>, return <em>the length of the shortest <strong>clear path</strong> in the matrix</em>. If there is no clear path, return <code>-1</code>.</p>

<p>A <strong>clear path</strong> in a binary matrix is a path from the <strong>top-left</strong> cell (i.e., <code>(0, 0)</code>) to the <strong>bottom-right</strong> cell (i.e., <code>(n - 1, n - 1)</code>) such that:</p>

<ul>
	<li>All the visited cells of the path are <code>0</code>.</li>
	<li>All the adjacent cells of the path are <strong>8-directionally</strong> connected (i.e., they are different and they share an edge or a corner).</li>
</ul>

<p>The <strong>length of a clear path</strong> is the number of visited cells of this path.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/18/example1_1.png" style="width: 500px; height: 234px;" />
<pre>
<strong>Input:</strong> grid = [[0,1],[1,0]]
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/18/example2_1.png" style="height: 216px; width: 500px;" />
<pre>
<strong>Input:</strong> grid = [[0,0,0],[1,1,0],[1,1,0]]
<strong>Output:</strong> 4
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> grid = [[1,0,0],[1,1,0],[1,1,0]]
<strong>Output:</strong> -1
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == grid.length</code></li>
	<li><code>n == grid[i].length</code></li>
	<li><code>1 &lt;= n &lt;= 100</code></li>
	<li><code>grid[i][j] is 0 or 1</code></li>
</ul>

class Solution {
    bool isValidIndex(int i, int j, int m, int n) {
        bool isInvalid = (i < 0) || (i >= m) || (j < 0)  || (j >= n);
        return !isInvalid;
    }
public:
    int shortestPathBinaryMatrix(vector<vector<int>>& grid) {
        queue<pair<int, int>> q;
        unordered_set<int> isVisited;

        int m = grid.size();
        int n = grid[0].size();

        q.push(pair(0, 0));
        int dist = 0;

        while(!q.empty()) {
            int entries = q.size();
            for(int x=0; x<entries; x++) {
                pair<int, int> index = q.front();
                q.pop();
                int i = index.first;
                int j = index.second;
                int visitedKey = i * n + j;
                if(isVisited.find(visitedKey)!=isVisited.end() || grid[i][j]==1) {
                    continue;
                }
                
                if(i == m-1 && j == n-1) {
                    return dist+1;
                }
                
                isVisited.insert(visitedKey);
                if(isValidIndex(i+1, j, m, n)) q.push(pair(i+1, j));
                if(isValidIndex(i-1, j, m, n)) q.push(pair(i-1, j));
                if(isValidIndex(i, j+1, m, n)) q.push(pair(i, j+1));
                if(isValidIndex(i, j-1, m, n)) q.push(pair(i, j-1));
                if(isValidIndex(i+1, j+1, m, n)) q.push(pair(i+1, j+1));
                if(isValidIndex(i+1, j-1, m, n)) q.push(pair(i+1, j-1));
                if(isValidIndex(i-1, j+1, m, n)) q.push(pair(i-1, j+1));
                if(isValidIndex(i-1, j-1, m, n)) q.push(pair(i-1, j-1));
            }
            dist++;
        }

        return -1;
    }
};
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