solve: pacific atlantic water flow

Author: sounmind

pacific-atlantic-water-flow/evan.py

@@ -0,0 +1,62 @@
+from typing import List
+
+
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        # If the input is empty, return an empty list
+        if not heights:
+            return []
+
+        # Get the number of rows and columns in the grid
+        row_length, col_length = len(heights), len(heights[0])
+
+        # Initialize 2D arrays to track cells reachable by Pacific and Atlantic oceans
+        pacific_reachable = [
+            [False for _ in range(col_length)] for _ in range(row_length)
+        ]
+        atlantic_reachable = [
+            [False for _ in range(col_length)] for _ in range(row_length)
+        ]
+
+        # Define a depth-first search (DFS) function
+        def dfs(row, col, reachable, prev_height):
+            # Terminate if the cell is out of bounds, already visited, or has a lower height
+            if (
+                row < 0
+                or col < 0
+                or row >= row_length
+                or col >= col_length
+                or reachable[row][col]
+                or heights[row][col] < prev_height
+            ):
+                return
+
+            # Mark the current cell as reachable
+            reachable[row][col] = True
+
+            # Perform DFS in all four directions
+            for delta_row, delta_column in [(1, 0), (-1, 0), (0, 1), (0, -1)]:
+                dfs(row + delta_row, col + delta_column, reachable, heights[row][col])
+
+        # Start DFS from the edges of the grid that are adjacent to the Pacific and Atlantic oceans
+        for i in range(row_length):
+            dfs(i, 0, pacific_reachable, heights[i][0])  # Pacific side
+            dfs(
+                i, col_length - 1, atlantic_reachable, heights[i][col_length - 1]
+            )  # Atlantic side
+
+        for j in range(col_length):
+            dfs(0, j, pacific_reachable, heights[0][j])  # Pacific top
+            dfs(
+                row_length - 1, j, atlantic_reachable, heights[row_length - 1][j]
+            )  # Atlantic bottom
+
+        result = []
+
+        # Collect cells that can reach both the Pacific and Atlantic oceans
+        for r in range(row_length):
+            for c in range(col_length):
+                if pacific_reachable[r][c] and atlantic_reachable[r][c]:
+                    result.append([r, c])
+
+        return result