Pacific Atlantic Water Flow

Given an m x n matrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges. Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower. Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean. Tags: Graph

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Solution

class Solution:
    def pacificAtlantic(self, matrix: List[List[int]]) -> List[List[int]]:
        if not matrix:
            return []
        
        m = len(matrix)
        n = len(matrix[0])
        
        p_visited = [[False for c in range(n)] for r in range(m)]
        a_visited = [[False for c in range(n)] for r in range(m)]
        res = []
        
        for r in range(m):
            self.dfs(matrix, r, 0, p_visited, m, n)
            self.dfs(matrix, r, n - 1, a_visited, m, n)
        
        for c in range(n):
            self.dfs(matrix, 0, c, p_visited, m, n)
            self.dfs(matrix, m - 1, c, a_visited, m, n)
        
        for r in range(m):
            for c in range(n):
                if p_visited[r][c] and a_visited[r][c]:
                    res.append([r, c])
        return res
    
    def dfs(self, matrix, r, c, visited, m, n):
        directions = [(-1, 0), (1, 0), (0, -1), (0, 1)]
        visited[r][c] = True
        
        for direction in directions:
            next_r = r + direction[0]
            next_c = c + direction[1]
            
            if next_r < 0 or next_r >= m:
                continue
            if next_c < 0 or next_c >= n:
                continue
            if visited[next_r][next_c]:
                continue
            if matrix[next_r][next_c] < matrix[r][c]:
                continue
            
            self.dfs(matrix, next_r, next_c, visited, m, n)