"""
https://leetcode.com/problems/number-of-islands/description/
Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.
An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
"""
from typing import List
DIRECTIONS = [[0, 1], [0, -1], [-1, 0], [1, 0]]
def count(arr: List[List[int]]) -> int:
ROWS, COLS = len(arr), len(arr[0])
visited = set()
islands = 0
def inbounds(row, col):
return row in range(ROWS) and col in range(COLS)
def land(row, col):
return arr[row][col] != "0"
def seen(row, col):
return (row, col) in visited
def dfs(r, c):
if not inbounds(r, c) or not land(r, c) or seen(r, c):
return
# element is inbounds, on land and not visited
visited.add((r, c))
for dx, dy in DIRECTIONS:
dfs(r + dx, c + dy)
for r in range(ROWS):
for c in range(COLS):
if land(r, c) and not seen(r, c):
islands += 1
dfs(r, c)
return islands