Update README.md

graphs/README.md

@@ -208,3 +208,56 @@ def iterative_dfs(graph, v, discovered):
                 stack.append(u)
 ```
   
+<br>
+
+---
+
+### matrix bfs
+
+<br>
+
+* given an `m x n` grid rooms initialized with these three possible values:
+    * -1 A wall or an obstacle.
+    * 0 A gate.
+    * `INF` Infinity means an empty room (`2^31 - 1 = 2147483647` to represent `INF`)
+
+* fill empty room with the distance to its nearest gate. if it is impossible to reach a gate, it should be filled with `INF`.
+
+
+```python
+
+
+def matrix_bfs(rooms) -> None:
+        
+        m = len(rooms)
+        if m == 0:
+           return rooms
+        n = len(rooms[0])
+        
+        GATE = 0
+        EMPTY = 2147483647
+        DIRECTIONS = ((1, 0), (-1, 0), (0, 1), (0, -1))
+
+        queue = collections.deque()
+
+        for i in range(m):
+            for j in range(n):
+                
+                if rooms[i][j] == GATE:
+                    q.append((i, j))
+
+        while queue:
+
+            row, col = queue.popleft()
+
+            for (x, y) in DIRECTIONS:
+
+                r = row + x
+                c = col + y
+
+                if (r < 0) or (c < 0) or (r >= m) or (c >= n) or rooms[r][c] != EMPTY:
+                    continue
+
+                rooms[r][c] = rooms[row][col] + 1
+                queue.append((r, c))            
+```