LeetCode 417 - Pacific Atlantic Water Flow - 2023.01.03 solution

class Solution:
    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
        """ How to solve it: Iterate through each cell of the four board edges.
        For each cell on a given ocean border, run DFS to find all cells that can flow
        to it.  Add those positions to a can_reach_pacific (or atlantic) set.
        Then just return the intersection of the two sets.
        """
        ROWS = len(heights)
        COLS = len(heights[0])

        top_row = [(0, i) for i in range(COLS)]
        left_col = [(i, 0) for i in range(ROWS)]
        bottom_row = [(ROWS-1, i) for i in range(COLS)]
        right_col = [(i, COLS-1) for i in range(ROWS)]

        can_reach_pacific = set(top_row + left_col)
        can_reach_atlantic = set(right_col + bottom_row)

        def add_neighbors_to_set(p, set_to_use):
            print('add_neighbors_to_set: {}'.format(p))
            mov_dirs = [
                [0, 1],
                [0, -1],
                [1, 0],
                [-1, 0],
            ]
            neighbors = [(p[0] + d[0], p[1] + d[1]) for d in mov_dirs]
            for neighbor in neighbors:
                if neighbor[0] < 0 or neighbor[0] >= ROWS or \
                   neighbor[1] < 0 or neighbor[1] >= COLS or \
                   heights[neighbor[0]][neighbor[1]] < heights[p[0]][p[1]] or \
                   neighbor in set_to_use:
                   continue
                else:
                    set_to_use.add(neighbor)
                    add_neighbors_to_set(neighbor, set_to_use)
        print('add_neighbors_to_set for pacific')
        for pos in top_row + left_col:
            print('for loop: {}'.format(pos))
            add_neighbors_to_set(pos, can_reach_pacific)
        print('add_neighbors_to_set for atlantic')
        for pos in bottom_row + right_col:
            print('for loop: {}'.format(pos))
            add_neighbors_to_set(pos, can_reach_atlantic)
        # print(': {}'.format())
        return can_reach_pacific.intersection(can_reach_atlantic)