<p>There is a <strong>1-based</strong> binary matrix where <code>0</code> represents land and <code>1</code> represents water. You are given integers <code>row</code> and <code>col</code> representing the number of rows and columns in the matrix, respectively.</p>

<p>Initially on day <code>0</code>, the <strong>entire</strong> matrix is <strong>land</strong>. However, each day a new cell becomes flooded with <strong>water</strong>. You are given a <strong>1-based</strong> 2D array <code>cells</code>, where <code>cells[i] = [r<sub>i</sub>, c<sub>i</sub>]</code> represents that on the <code>i<sup>th</sup></code> day, the cell on the <code>r<sub>i</sub><sup>th</sup></code> row and <code>c<sub>i</sub><sup>th</sup></code> column (<strong>1-based</strong> coordinates) will be covered with <strong>water</strong> (i.e., changed to <code>1</code>).</p>

<p>You want to find the <strong>last</strong> day that it is possible to walk from the <strong>top</strong> to the <strong>bottom</strong> by only walking on land cells. You can start from <strong>any</strong> cell in the top row and end at <strong>any</strong> cell in the bottom row. You can only travel in the<strong> four</strong> cardinal directions (left, right, up, and down).</p>

<p>Return <em>the <strong>last</strong> day where it is possible to walk from the <strong>top</strong> to the <strong>bottom</strong> by only walking on land cells</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/07/27/1.png" style="width: 624px; height: 162px;" />
<pre>
<strong>Input:</strong> row = 2, col = 2, cells = [[1,1],[2,1],[1,2],[2,2]]
<strong>Output:</strong> 2
<strong>Explanation:</strong> The above image depicts how the matrix changes each day starting from day 0.
The last day where it is possible to cross from top to bottom is on day 2.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/07/27/2.png" style="width: 504px; height: 178px;" />
<pre>
<strong>Input:</strong> row = 2, col = 2, cells = [[1,1],[1,2],[2,1],[2,2]]
<strong>Output:</strong> 1
<strong>Explanation:</strong> The above image depicts how the matrix changes each day starting from day 0.
The last day where it is possible to cross from top to bottom is on day 1.
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/07/27/3.png" style="width: 666px; height: 167px;" />
<pre>
<strong>Input:</strong> row = 3, col = 3, cells = [[1,2],[2,1],[3,3],[2,2],[1,1],[1,3],[2,3],[3,2],[3,1]]
<strong>Output:</strong> 3
<strong>Explanation:</strong> The above image depicts how the matrix changes each day starting from day 0.
The last day where it is possible to cross from top to bottom is on day 3.
</pre>

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

<ul>
	<li><code>2 &lt;= row, col &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>4 &lt;= row * col &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>cells.length == row * col</code></li>
	<li><code>1 &lt;= r<sub>i</sub> &lt;= row</code></li>
	<li><code>1 &lt;= c<sub>i</sub> &lt;= col</code></li>
	<li>All the values of <code>cells</code> are <strong>unique</strong>.</li>
</ul>

class Solution:
    def latestDayToCross(self, row: int, col: int, cells: List[List[int]]) -> int:
        DIR = [0, 1, 0, -1, 0]

        def canWalkFromTopToBottom(dayAt):
            grid = [[0] * col for _ in range(row)]  # Create grid in the `dayAt` th date
            for i in range(dayAt):
                r, c = cells[i]
                grid[r - 1][c - 1] = 1  # Mark as water

            bfs = deque([])
            for c in range(col):
                if grid[0][c] == 0:  # Add all valid start cells in the top row
                    bfs.append((0, c))
                    grid[0][c] = 1  # Mark as visited

            while bfs:
                r, c = bfs.popleft()
                if r == row - 1: return True  # Reach to bottom -> Return Valid
                for i in range(4):
                    nr, nc = r + DIR[i], c + DIR[i + 1]
                    if nr < 0 or nr == row or nc < 0 or nc == col or grid[nr][nc] == 1: continue
                    grid[nr][nc] = 1  # Mark as visited
                    bfs.append((nr, nc))
            return False

        left, right = 1, len(cells)
        ans = 0
        while left <= right:
            mid = (left + right) // 2
            if canWalkFromTopToBottom(mid):
                ans = mid  # Update current answer so far
                left = mid + 1  # Try to find a larger day
            else:
                right = mid - 1  # Try to find a smaller day
        return ans
1956