adding updates

src/my_project/interviews/google_top_exercises/round_1/27_pacific_atlantic_water_flow.py

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+from typing import List
+from collections import deque
+
+
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        """Reverse DFS from each ocean's border, then intersect the two sets.
+
+        Instead of asking "can this cell reach an ocean?" for every cell, we start
+        at the ocean edges and walk uphill (neighbor height >= current height).
+        Every cell reached that way can flow back down into that ocean.
+        """
+
+        if not heights or not heights[0]:
+            return []
+
+        rows, cols = len(heights), len(heights[0])
+        pacific, atlantic = set(), set()
+
+        def dfs(r: int, c: int, visited: set, prev_height: int):
+            if (r < 0 or r >= rows or c < 0 or c >= cols or
+                    (r, c) in visited or heights[r][c] < prev_height):
+                return
+
+            visited.add((r, c))
+            height = heights[r][c]
+
+            dfs(r - 1, c, visited, height)
+            dfs(r + 1, c, visited, height)
+            dfs(r, c - 1, visited, height)
+            dfs(r, c + 1, visited, height)
+
+        # Top row -> Pacific, bottom row -> Atlantic
+        for c in range(cols):
+            dfs(0, c, pacific, heights[0][c])
+            dfs(rows - 1, c, atlantic, heights[rows - 1][c])
+
+        # Left column -> Pacific, right column -> Atlantic
+        for r in range(rows):
+            dfs(r, 0, pacific, heights[r][0])
+            dfs(r, cols - 1, atlantic, heights[r][cols - 1])
+
+        return [[r, c] for r, c in pacific & atlantic]
+
+    def pacificAtlantic_BFS(self, heights: List[List[int]]) -> List[List[int]]:
+        """Same idea with multi-source BFS - avoids Python's recursion limit."""
+
+        if not heights or not heights[0]:
+            return []
+
+        rows, cols = len(heights), len(heights[0])
+
+        def bfs(starts: List[tuple]) -> set:
+            visited = set(starts)
+            q = deque(starts)
+
+            while q:
+                r, c = q.popleft()
+                for dr, dc in [(0, 1), (1, 0), (0, -1), (-1, 0)]:
+                    nr, nc = r + dr, c + dc
+                    if (0 <= nr < rows and 0 <= nc < cols and
+                            (nr, nc) not in visited and
+                            heights[nr][nc] >= heights[r][c]):
+                        visited.add((nr, nc))
+                        q.append((nr, nc))
+
+            return visited
+
+        pacific_starts = [(0, c) for c in range(cols)] + [(r, 0) for r in range(rows)]
+        atlantic_starts = ([(rows - 1, c) for c in range(cols)] +
+                           [(r, cols - 1) for r in range(rows)])
+
+        return [[r, c] for r, c in bfs(pacific_starts) & bfs(atlantic_starts)]