add-solution: pacific-atlantic-water-flow

Author: dusunax

pacific-atlantic-water-flow/dusunax.py

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+'''
+# 417. Pacific Atlantic Water Flow
+
+## Time Complexity: O(n * m)
+- dfs is called for each cell in the grid, and each cell is visited once.
+
+## Space Complexity: O(n * m)
+- pacific and atlantic sets store the cells that can flow to the pacific and atlantic oceans respectively.
+'''
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        if len(heights) == 1 and len(heights[0]) == 1:
+            return [[0, 0]]
+        
+        max_row, max_col = len(heights), len(heights[0])
+        pacific, atlantic = set(), set()
+        directions = [(1, 0), (0, 1), (-1, 0), (0, -1)]
+
+        def dfs(r, c, visited, prev_height):
+            out_of_bound = r < 0 or c < 0 or r >= max_row or c >= max_col
+            if out_of_bound:
+                return
+
+            current = heights[r][c]
+            is_visited = (r, c) in visited
+            is_uphill = current < prev_height
+            if is_visited or is_uphill:
+                return
+            
+            visited.add((r, c))
+
+            for dr, dc in directions:
+                dfs(r + dr, c + dc, visited, current)
+
+        for r in range(max_row):
+            dfs(r, 0, pacific, heights[r][0]) # left
+            dfs(r, max_col - 1, atlantic, heights[r][max_col - 1]) # right
+        for c in range(max_col):
+            dfs(0, c, pacific, heights[0][c]) # top
+            dfs(max_row - 1, c, atlantic, heights[max_row - 1][c]) # bottom
+
+        return list(pacific & atlantic)