Pacific Atlantic Water Flow – Solution & Complexity

def pacific_atlantic(heights: list[list[int]]) -> list[list[int]]:
    rows, cols = len(heights), len(heights[0])
    def reach(starts):
        seen, stack = set(starts), list(starts)
        while stack:
            r, c = stack.pop()
            for dr, dc in ((1,0),(-1,0),(0,1),(0,-1)):
                nr, nc = r + dr, c + dc
                if 0 <= nr < rows and 0 <= nc < cols and (nr,nc) not in seen and heights[nr][nc] >= heights[r][c]:
                    seen.add((nr,nc)); stack.append((nr,nc))
        return seen
    pacific = reach([(0,c) for c in range(cols)] + [(r,0) for r in range(rows)])
    atlantic = reach([(rows-1,c) for c in range(cols)] + [(r,cols-1) for r in range(rows)])
    return [[r,c] for r in range(rows) for c in range(cols) if (r,c) in pacific & atlantic]