<p>You are given <code>row x col</code> <code>grid</code> representing a map where <code>grid[i][j] = 1</code> represents&nbsp;land and <code>grid[i][j] = 0</code> represents water.</p>

<p>Grid cells are connected <strong>horizontally/vertically</strong> (not diagonally). The <code>grid</code> is completely surrounded by water, and there is exactly one island (i.e., one or more connected land cells).</p>

<p>The island doesn&#39;t have &quot;lakes&quot;, meaning the water inside isn&#39;t connected to the water around the island. One cell is a square with side length 1. The grid is rectangular, width and height don&#39;t exceed 100. Determine the perimeter of the island.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img src="https://assets.leetcode.com/uploads/2018/10/12/island.png" style="width: 221px; height: 213px;" />
<pre>
<strong>Input:</strong> grid = [[0,1,0,0],[1,1,1,0],[0,1,0,0],[1,1,0,0]]
<strong>Output:</strong> 16
<strong>Explanation:</strong> The perimeter is the 16 yellow stripes in the image above.
</pre>

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

<pre>
<strong>Input:</strong> grid = [[1]]
<strong>Output:</strong> 4
</pre>

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

<pre>
<strong>Input:</strong> grid = [[1,0]]
<strong>Output:</strong> 4
</pre>

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

<ul>
	<li><code>row == grid.length</code></li>
	<li><code>col == grid[i].length</code></li>
	<li><code>1 &lt;= row, col &lt;= 100</code></li>
	<li><code>grid[i][j]</code> is <code>0</code> or <code>1</code>.</li>
	<li>There is exactly one island in <code>grid</code>.</li>
</ul>

class Solution {
private:
    vector<int> stepCol = {1, 0, -1, 0};
    vector<int> stepRow = {0, 1, 0, -1};
    bool valid(int row, int col, auto& grid) {
        return (row >=0 && row < grid.size() && col >=0 && col < grid[0].size());
    }
public:
    int islandPerimeter(vector<vector<int>>& grid) {
        queue<pair<int, int>> q;
        int result = 0;
        int nRow = grid.size();
        int nCol = grid[0].size();
        vector<vector<bool>> visited(nRow, vector<bool>(nCol, false));
        // find first island loc.
        for (int i = 0; i < nRow; i++) {
            for (int j = 0; j < nCol; j++) {
                if (grid[i][j] == 1) {
                    q.push({i, j});
                    visited[i][j] = true;
                    break;
                } 
            }
            if (!q.empty()) break;
        }
        while(!q.empty()) {
            pair<int, int> apair = q.front(); q.pop();
            int curRow = apair.first;
            int curCol = apair.second;
            for (int i = 0; i < 4; i++) {
                int nextRow = curRow + stepRow[i];
                int nextCol = curCol + stepCol[i];
                if (!valid(nextRow, nextCol, grid) || grid[nextRow][nextCol] == 0) {
                    result++;
                }
                if (valid(nextRow, nextCol, grid)
                    && visited[nextRow][nextCol] == false
                    && grid[nextRow][nextCol] == 1) {
                    q.push({nextRow, nextCol});
                    visited[nextRow][nextCol] = true;
                }
            }
        }
        return result;
    }
};
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